Lights and signalling: being seen underwater and at the surface.

Dive lights

A Scuba Diving magazine buyer's guide hosted by PADI (March 2020) says lights are what make night diving and wreck penetration possible. The guide names three broad types:

• Primary lights: the main light a diver carries.
• Backup lights: compact second lights.
• Video or imaging lights: wide-beam lights for filming and photography.

Beams

The same guide describes narrow beams of about 8 to 10 degrees for reaching into crevices and dark spaces, adjustable beams of roughly 12 to 75 degrees, and wide beams near 90 degrees for video.

Batteries, depth and burn time

Many lights in the guide run on rechargeable lithium-ion cells, often the 18650 size; others take single-use lithium CR123A cells or C-size cells. Most of the lights reviewed were rated by their makers to about 100 m, and one to 150 m, with burn times from about two hours on high to about eight on low. Those are maker ratings as the magazine reported them in 2020, not tests of any product today.

Features the guide names include several power levels, SOS flash modes, locks to stop the light switching on by accident, and shrouds that keep the beam from dazzling other divers. Lithium batteries also bring airline rules, covered on travelling with dive gear.

Surface signalling devices

PADI recommends carrying at least two surface signalling devices, one you can hear and one you can see, and says more adds safety. Its suggested common starter kit is a whistle plus a signal tube or delayed surface marker buoy, adding a mirror and an air horn for remote destinations.

Visual devices

Signal tube

PADI describes an inflatable tube, bright orange or yellow, that rises more than a metre above the water and rolls up small enough for a BCD pocket. PADI says such tubes are required in many areas.

Delayed surface marker buoy (DSMB)

Similar to a signal tube but attached to a line and reel, so it can be sent up from underwater as well as used at the surface, per PADI. DAN says DSMBs can mark a diver's position before surfacing but need training, because handling the line while inflating is tricky, and that small marker buoys can be hidden by waves.

Signal mirror

PADI says a mirror reflects sunlight and can attract aircraft. DAN adds that it needs strong direct sun and helps little on overcast days or at night.

Lights and flashers

PADI and DAN both say these work best at night. DAN notes that SOS modes help rescuers find a diver after dark.

Audible devices

PADI calls the whistle the most common audible device and suggests clipping it to the BCD inflator hose. Air horns run on gas from the cylinder and are much louder, which is why PADI calls a whistle a sensible backup. DAN points out that integrated air horns add possible failure points to the BCD and need gas in the cylinder to work at the surface.

Personal locator beacons

DAN describes personal locator beacons as satellite SOS devices, some able to send messages, and says their batteries need regular checking.

Dive flags

Boats and divers use flags to warn other vessels that divers are in the water, and the rules on which flag counts are not uniform. Using the United States as an example:

• A boater-education guide for Washington State says a vessel restricted in its ability to manoeuvre because of diving must show a rigid blue-and-white International Code Flag A, at least one metre high and visible all round. It says other boats should keep well clear, preferably at least 200 feet away.
• The other flag in use is red with a white diagonal stripe. A US Coast Guard notice, which we saw only as a quotation on a flags website, calls it an unofficial signal adopted by custom to protect divers in the water and says it is wrong to think the Alpha flag replaced it.

Sources disagree on the red-and-white flag's legal standing. The Washington guide says it does not meet state and federal display requirements, while other sources say state law or regulation requires it in virtually every US state. Rules vary by state, so the local boating authority is the place to check.