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A red flag with a white diagonal stripe is a diver-down flag. It warns that a diver is in the water nearby, possibly below the surface and hard to see. If you are operating a boat, slow as conditions require, keep well clear, and maintain a dedicated lookout. The exact flag design, display method, diver-to-flag distance, and boat-clearance rule depend on the state or country where you are boating.
What does a red and white diver-down flag mean?
The red-and-white flag identifies an area where someone is engaged in diving. The red field is normally square or rectangular, with a contrasting white diagonal stripe running across it. It is a warning signal, not a marker that makes the diver visible or protects the entire waterway automatically.
A diver can be several feet below the surface, obscured by glare, waves, vegetation, bubbles, or boat traffic. Treat the flag as an indication that a person may surface unexpectedly in the marked area. Stay alert even when you cannot see a diver, float, bubbles, or a support boat.
Pennsylvania Fish and Boat Commission material labels the red-and-white signal “Diver down.” California boating education material describes it as indicating a person engaged in diving nearby and tells other boats to stay clear. Those descriptions express the common meaning, while the legal details are set locally.
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What does a red flag with a white stripe on a boat mean?
In the usual recreational-boating context, it means divers are in the immediate area. The flag may be mounted on a boat, a pole, or a surface float, depending on local law and the dive setup. It does not mean the boat is displaying a general distress signal, and it is not the same as the blue-and-white maritime Code Flag Alpha.
Do not infer a worldwide minimum distance from the flag alone. A state may specify the flag’s dimensions, stiffness, height above the water, support, required location, and how close a diver must remain. Another jurisdiction may use different requirements or terminology.
What should a boater do when seeing a diver-down flag?
- Reduce speed appropriately. Avoid creating a wake or continuing at a speed that prevents you from stopping or steering safely. In some jurisdictions, a special no-wake or headway rule applies within a specified distance.
- Give the marked area a wide berth. Use the local clearance distance rather than assuming the flag is directly above the diver. The diver may be swimming away from the float or boat.
- Assign a lookout. Scan the surface around the flag and along your intended course. A person should watch for a diver surfacing while the operator concentrates on navigation.
- Keep the propeller and hull away from the signal. Do not pass between a diver-down flag and its support boat or float unless local rules clearly permit it and you can do so without risk.
- Follow the waterway’s rules. Check the relevant state boating agency, harbor authority, or national maritime authority before operating near a dive site.
Visibility, current, wind, traffic, and the diver’s position can make a nominal legal minimum inadequate as a practical safety margin. When in doubt, wait or route around the area.
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What should a diver do when displaying the flag?
- Use the construction and support required by the jurisdiction where the dive occurs.
- Place the flag where approaching boaters can see it from likely directions, without allowing it to collapse or lie flat on the water.
- Remain within any legally specified distance of the flag, float, or support vessel.
- Brief the boat operator and surface tender on the local clearance rule before entering the water.
- Lower or remove the signal when the diving operation ends if local rules require that it not remain displayed unnecessarily.
A diver-down flag is an aid to recognition, not a substitute for a surface tender, lookout, audible warning, or sensible site selection. Use additional precautions in busy channels, near ramps, and where a flag can be hidden by waves or shoreline structures.
What are Florida’s diver-down flag rules?
Florida provides a useful example of how specific local rules can be. Florida describes the diver-down symbol as a red rectangle or square with a white diagonal stripe. Its published guidance says boaters must make a reasonable effort to remain at least 300 feet from a displayed diver-down device in open water and at least 100 feet away in rivers, inlets, or navigation channels.
Florida guidance also states that a vessel approaching within those limits generally must proceed no faster than necessary to maintain headway and steerageway. Divers must make a reasonable effort to stay within 300 feet of the device in open water and within 100 feet in rivers, inlets, or navigation channels. These figures are Florida requirements and guidance, not a universal maritime standard.
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Because statutes and agency guidance can change, verify the current Florida Fish and Wildlife Conservation Commission material and the applicable statute before relying on those distances for a particular trip.
How do Massachusetts, Pennsylvania, and California differ?
| Jurisdiction | Documented requirement or guidance | How to use it |
|---|---|---|
| Massachusetts | State law requires a rigidly supported red flag at least 12 by 15 inches, with a white diagonal stripe, displayed on a boat or surface float at least three feet above the water. | These are Massachusetts specifications, not a default size or height for every location. |
| Pennsylvania | The Fish and Boat Commission identifies “Diver down” and blue-and-white “Code Flag Alpha” as separate diving signs and advises boats to remain at least 100 feet from areas displaying them. | The 100-foot figure is Pennsylvania guidance and should not be presented as a nationwide rule. |
| California | Boating education material says the red-and-white flag indicates a person engaged in diving in the immediate area and warns other boats to stay clear. | Use the warning as an instruction to avoid the area, then check the applicable California rule for exact operating requirements. |
The differences illustrate why a commercially available flag that looks correct may still fail a local requirement for size, rigidity, height, or mounting.
Is the diver-down flag the same as Code Flag Alpha?
No. The traditional diver-down flag is red with a white diagonal stripe. Code Flag Alpha is blue and white and has a swallowtail shape. They are related to diving but serve different signaling contexts.
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The U.S. Coast Guard Navigation Center’s Local Notice to Mariners 13/2025 specifically rejects the idea that Code Flag Alpha replaced the traditional diver’s flag: “This impression is incorrect.” The statement addresses a mistaken belief that every diver must substitute Alpha for the red-and-white flag.
Under the Coast Guard navigation rules, Alpha is associated with a vessel engaged in diving operations when that vessel is restricted in its ability to maneuver. A qualifying vessel may need the prescribed all-round lights or shapes. When the vessel’s size makes the full display impracticable, the rules describe a rigid replica of Code Flag A at least one meter high and visible all around.
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That technical requirement applies to the specified restricted-maneuverability vessel context. It is not a general instruction for every recreational diver to fly a one-meter Alpha flag instead of displaying a red-and-white diver-down signal. Determine which navigation-rule category applies to the vessel and operation, and follow both federal and local requirements.
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How can you identify the signal correctly?
Check the colors and stripe
Look for a red field with a single white diagonal stripe. The flag is commonly rectangular or square. A faded, upside-down, or partially hidden flag may be difficult to distinguish, so confirm it from more than one angle when possible.
Look for the supporting setup
The signal may be on a dive boat, a pole, or a float. Its presence should prompt you to search for a tender, anchored vessel, swim platform, bubbles, or a diver in the water.
Do not rely on flag color alone
Blue-and-white Alpha, an ordinary red pennant, a racing signal, and emergency markings have different meanings. If the shape or color is unclear, take the conservative action: slow down, keep away, and identify the local rule before approaching.
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- Assuming the diver is directly underneath. Divers can travel away from a float. Keep clear of the broader marked operation, not just the pole.
- Treating a legal minimum as a recommended target. Current, visibility, vessel size, and traffic may require more room than the minimum.
- Using one state’s specification everywhere. Florida’s 100-foot and 300-foot figures, Massachusetts’ 12-by-15-inch flag rule, and Pennsylvania’s 100-foot guidance each have a defined local scope.
- Replacing the red-and-white flag with Alpha without checking the context. Alpha concerns specified vessels restricted in their ability to maneuver; it does not erase the traditional diver-down signal.
- Leaving the flag unattended after diving ends. An outdated warning can cause unnecessary avoidance or confusion. Follow the local rule for removing it.
- Passing close at cruising speed. Even if you see no diver, propellers and wakes can injure someone who surfaces laterally or is hidden by glare.
Which rule should you check before a dive?
- Identify the jurisdiction: state waters, a federal channel, an inland lake, or another country’s waters may involve different authorities.
- Read the current boating and diving rule for that location, including definitions of “diver-down device,” “open water,” channel, and restricted maneuverability.
- Confirm required flag dimensions, rigidity, support, height, visibility, and mounting point.
- Confirm the boat-clearance distance, speed restriction, and the diver’s permitted distance from the flag.
- Brief everyone aboard and use a lookout or tender who understands the signal and emergency plan.
If a state law, harbor rule, or navigation regulation is more specific than general boating advice, follow the applicable legal requirement. For international or unfamiliar waters, consult the local maritime authority rather than importing a U.S. state rule.
Bottom line for boaters and divers
A red flag with a white diagonal stripe means a diver is in the nearby water. Boat operators should slow as necessary, maintain a lookout, and stay well clear under the local rule. Divers should display the signal in the required form and remain within the permitted distance. Code Flag Alpha is a separate blue-and-white maritime signal for specified restricted-maneuverability vessels; it has not replaced the traditional diver-down flag.
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