Any specific concern wrt water?

I'm looking at the question in this light: How are motorcycles when it comes to water?  IMO, too general and the only thought would be - metal bits can oxidize which likely doesn't help in your q.

Additionally, will it have steel or synthetic line?


edit... looking at your link more, two items pop up:
1) How difficult is it to get to the clutch lever? If not easy, which can be the case on some hidden hitches, you'll be slowly de-spooling.

2) If synthetic, I'm a fan of these end of line items as they fully contain the line to keep UV from hitting it at that bend in the line. UV degrades plastic, one of the weaknesses of synth wish is otherwise fairly impressive.
http://www.pelfreybilt.com/?_escaped_fragment_=product/prd1/3549428471/factor-55-prolink#!product/prd1/3549428471/factor-55-prolink

They are "reasonably" sealed but are not waterproof. On my Warn XD9000 (a typical designed unit), the motor's housing (I don't think) has any gasket or o-ring for water seal, though it would seem to be a fairly tight metal-to-metal mate.

I don't have any great info for you - perhaps someone else will chime in. I do know that used winches can have a decent amount of rust inside the motor housing and gear housing.

Google search maybe?

EDIT: One of the members on the 4x4 group compiled a bunch of good winch info. You might find something of interest:
http://www.frontrange4x4.com/forums/showthread.php?2134-WINCHES-Rebuilding-Parts-Information-Diagrams-Testing-Sites