Use any waterproof type grease.

Well by definition most greases are "waterproof", oil & water don't mix.


I'd recomend a "Marine" Grade grease, especially if you ride in water and/or mud a lot.  It'll help keep the water (and rust) out of the bearing surfaces as long as you keep them greased.

I use Sta-Lube marine grade, it's blue and you can get it by the tube at home depot for about $7 a tube.  from my experience it holds up better than other greases I've tried or cleaned out from previous owners.

You can also get Lucas Marine at home depot for about the same price.
http://www.homedepot.com/p/Lucas-Oil-14-5-oz-Marine-Grease-10320/202535871