Mine spends a bit of time underwater. And I have ruined one motor, and roasted a set of gears. I have learned a couple of key points here to make sure to limit future problems.
1) the drain hole for the motor must be pointed DOWN. This may be an issue if you have your winch mounted "foot forward" like on an ARB bumper. WARN assums a "foot down" installation, there fore the drain hole is situated with that in mind. The mounting instructions make no mention of special considerations for other mounting positions. The cover for the motor can be "clocked" just like the gear housing on the other end.
More on this very subject can be read here: http://www.fjcruiserforums.com/forums/misc-technical-discussion/33307-winch-question-3.html
2) give the drain hole time to do it's thing. Don't do a deep water crossing and then immediately use the winch. Give the drain time to drain. I now set a one hour rule for myself.
3) perform regular scheduled maintenance on the winch. I break mine down to parade rest now every 4 months to clean and repack all the gears (planetary in my case), and clean and inspect the motor.
I have heard that there is a company that makes water proofing kits for all the popular WARN winches and I am interested. One of these days I'll actually get around to doing the research on that. I'll keep you posted...