Brawn fully waterproof all their winches (one of their selling points).
For Warns - an alloy oringed drum cap seals infinitely better than the crappy stock plastic one.
Charging the motor with pressurised air helps keep the crud out too.
For the solenoid pack you can go hi-tech, like the Mad dentist (the master of Overkill - pun intended)
and retro-fit a Mosfet sealed solenoid replacement pack from a Warn 9.5XP or go low-tech and relocated the stock solenoid box up high in the cab/engine bay - etc.
I choose to leave mine up front for easy access to fix any problems and replace solenoids if required. thankfully the old-school metal-bodied solenoids found in early hi-mounts have good life unlike the lameo later model smaller plastic bodied ones.
btw - re your q on covers. Warn make both soft and hard covers but IMHO you'd be better off getting one made of heavy duty waterproof canvas if that's what you really want.
http://www.gowarn.com/protective-covers.asp
Then you've just gotta remember to remove it and clean any water than gets in after a really decent dunking.