That looks like an optic I'd like to play with!
Anyway, 360deg eh', for 2 miles? Glow toob is out then.
Ping-pong balls.
Get a quick detach mount to affix to your boat and make a boat head for just about any smaller diameter light. Cut a nice hole in the ball and silicone it to the head. Just put it inside enough so the silicone can do it's job. You will get a nice all around glow for sure.
This can also be done with a stack of those small clear plastic medicine cups. Silicone those together and attach them to the top of the bezel.
White plastic film canister with a small round mirror glued into the bottom would be excellent. Light would bounce from reflector to mirror and back and the only way out is the white plastic sides. Dang! I need to go find a small round mirror. Maybe an old CD?