The clearest indicators of scuba diving or snorkeling activity are dive flags displayed on the water’s surface, streams of bubbles rising from below, and specific equipment visible on nearby boats or swimmers. Whether you’re a boater trying to stay safe, studying for a maritime license, or just curious about what you’re seeing on the water, these signals all communicate the same thing: divers are in the area, and vessels need to keep their distance.
Dive Flags on the Surface
Two flags are used worldwide to signal diving activity, and recognizing them is essential for anyone on the water.
The Diver Down flag is the one you’ll see most often in U.S. and Caribbean waters. It’s a red rectangle with a white diagonal stripe running from the upper left corner to the lower right. This flag is flown from boats, floats, or buoys near a dive site, and it means divers are actively underwater in the immediate area. Depending on local regulations, other vessels are required to stay between 100 and 300 feet away from this flag.
The Alpha flag is the international version, used under the International Maritime Code. It’s blue and white, split diagonally. Rather than simply warning “divers below,” it carries a more specific legal meaning: the vessel displaying it is restricted in its ability to maneuver because of diving operations. Other boats are required to give way. You’ll see the Alpha flag more commonly on commercial dive boats and in international waters, though many boats display both flags at the same time to cover all bases.
Day Shapes on Vessels
Beyond flags, larger vessels engaged in underwater operations use standardized day shapes, which are black geometric forms hoisted where they’re most visible. A vessel restricted in its ability to maneuver, including one conducting dive operations, displays a black ball, a black diamond, and a black ball arranged vertically. If the vessel has an obstruction on one side (common during underwater work or dredging), it adds two black balls on the obstructed side and two black diamonds on the side where other boats may safely pass. These signals are part of international collision regulations and apply in all waters.
Bubbles Rising From Below
A steady stream of bubbles breaking the surface is one of the most obvious real-time signs of scuba diving. Scuba divers breathe compressed air from tanks through a regulator, and every exhaled breath releases a cluster of bubbles that rises to the surface. These bubbles appear in regular intervals, roughly matching the diver’s breathing rhythm, and they can be visible from a surprising distance on calm water.
Snorkelers don’t produce the same pattern. Because they breathe through a tube that stays at the surface, the only bubbles you’ll typically see are occasional splashes or a burst of water clearing the snorkel tube after a brief duck-dive. The difference is distinctive: scuba bubbles are continuous and rise from depth, while snorkeling produces only intermittent surface disturbance.
Equipment You Can Spot
The gear visible on a boat or in the water tells you exactly what kind of activity is happening. Scuba diving requires substantially more equipment than snorkeling, and most of it is easy to identify even from a distance.
Scuba divers carry air tanks on their backs (or mounted alongside their bodies), wear a buoyancy control device that looks like an inflatable vest, and breathe through a regulator attached to the tank by a thick hose. They also carry dive computers or gauges to monitor depth and remaining air supply. Their fins tend to be larger and more rigid than snorkeling fins, designed to move a diver plus 30 to 50 pounds of gear through the water efficiently.
Snorkeling gear, by contrast, is minimal: a mask, a breathing tube, and a pair of fins. No tanks, no vest, no hoses. A snorkeler’s entire kit fits in a small bag. If you see a boat loaded with cylinders, weight belts, and heavy-duty fins, that’s a dive operation. If you see people floating face-down at the surface with just a tube sticking up, that’s snorkeling.
Disturbance on the Seafloor
Diving and snorkeling also leave physical traces in the underwater environment. Divers swimming close to the bottom can stir up sediment with their fins, creating visible clouds of sand or silt that drift with the current. In shallow, clear water, you can sometimes see these sediment plumes from the surface or from a boat.
On coral reefs, the signs of recent diving activity can be more lasting. Improper fin kicks disturb bottom sediment and deposit it on coral, which can smother the organisms and trigger stress responses like excess mucus production. Broken coral tips, fin scrape marks on sandy patches, and settled sediment on reef structures all indicate that divers or snorkelers have recently been in the area. Research published in Frontiers in Marine Science found that repeated sediment disturbance from divers caused measurable physiological stress in coral species, reducing growth rates and increasing vulnerability to bleaching.
Physical Signs on the Person
After a dive, the divers themselves may show some telltale signs. The most common is a visible impression or redness around the face where the dive mask sealed against the skin, sometimes called “mask squeeze.” Wet hair, salt-crusted skin, and the faint smell of neoprene from a wetsuit are more casual giveaways.
More significant physical indicators point to pressure-related injuries. The most frequent diving injury is middle ear barotrauma, where pressure changes during descent affect the ear. A person experiencing this may report ear pain, muffled hearing, a sensation of fullness, ringing in the ears, or dizziness. These symptoms appearing shortly after someone has been in the water are a strong indicator of recent diving at depth, since snorkelers rarely experience barotrauma unless they’re making repeated deep breath-hold dives.
Decompression sickness, which occurs when dissolved gas forms bubbles in the body during ascent, produces its own set of recognizable signs. Joint pain (particularly in the shoulders and elbows), skin mottling or marbling, unusual fatigue, numbness or tingling, dizziness, and coordination problems are all characteristic. These symptoms typically appear within hours of surfacing and are exclusive to compressed-gas diving, not snorkeling. A person showing these signs after time on the water has almost certainly been scuba diving.
Putting the Signs Together
No single indicator works in isolation. A red-and-white flag on a buoy tells you divers are present. Bubbles confirm someone is actively underwater on compressed air. Tanks and regulators on a nearby boat confirm the type of activity. And physical signs on a person after they leave the water can confirm recent diving even when no other evidence is visible. For boaters, the practical takeaway is straightforward: if you see a dive flag or bubbles, slow down and give the area a wide berth. Divers surface unpredictably, and they can be nearly invisible at the waterline.