I used to own a 22' bowrider, a Bayliner.
Back in the day I used to keep a 4D Maglite in the boats glove box, and it got a fair bit of use for navigating to the mooring in the dark.
I put some reflective strips on our dock in reference points to guide me in safely.
This was 5 or so years ago so I was not privvy to some of the later flashlights which would have made night-nav a lot easier.
1. Plenty of throw. A 'Hound Dog' emitter would be perfect. Distances out on the water are decieving and that nice little bright light of yours will get swallowed by the darkness.
2. I don't think it needs to float. Are you mildly retarded or a Navy SEAL? If neither, then just don't drop your light. I never dropped mine and was out on the high seas at night during the remnants of Katrina.
3. Dedicated holder. If you're determined to go with a hand-held, then, like a bicycle mount, find a way to mount the light in the cockpit so you can orient it in the direction you want to project the beam and not have to hold on to it.
4. 4D Maglite with a throwy drop-in would work.
5. Do you need a handheld? A dedicated marine spotlight sounds like it would be more effective. If I'd kept my boat a few more years I would have purchased a marine spotlight and mounted it in the cockpit.
6. Headlamp.
Ideally, I would go with a combo of a marine spotlight that is fixed to the boat, and wear a headlamp for personal lighting. Tying ropes with one hand and completing the mooring process in the dark and choppy water is not fun. That is a situation where dropping your handheld is a real possibility.
I can't think of a situation where I'd want (although its what I did) to operate a boat in the dark with one hand and fuss around on the docks with ropes with one hand. I want hands free on the wheel and hands free on the dock.
Headlight + Fixed spotlight = Win.
Powerful handheld = Bandaid fix.