My favorite? The Ultrafire DV-S9!
Magnetic slider switch, so no rubber boot, seals, nothing to deal with. Any movables that have to go underwater always spell trouble to me, so a magnetic slider with no holes or seals or whatever gets a thumbs-up from me.
Tailcap has 3 beefy O-rings sealing it.
Only thing I’d make sure of is the front-glass. Dunk it in a bucket o’ water overnight to make sure nothing leaks. 
Takes a 26650, or 18650 with included bands. Bright, too. You’ll be scaring those crabs right into the buckets in no time…
It does have a slight parasitic drain, so you need to give it a ¼-turn or so to lock it out when not being used for a while.