You'll have to excuse the background noises, I forgot to mute the microphone.
The tiny black box you'll see mounted on the front plate is the actual flasher unit itself, made by ACTion Electronics, details of the unit HERE. It's a tiny little unit, but not waterproof, so I filled the black box with epoxy resin to totally seal the whole thing. I've just tested it in the sink and it's now 100% watertight and works perfectly underwater. The LED is a 10mm diameter HiViz Ultra Brite from the Component Shop.
The whole set up is run from a 9volt PP3 battery which is inside it's own little black box, and sealed with Sellotape Diamond tape, this is 100% watertight tape, easy to apply to seal the box and easy to remove to change the battery.