Use a waterproof grease that you can get at the auto parts store. It is commonly used in boat trailer wheel bearings.
Zerk fittings are not a good idea in this application in my opinion. They invite overpressuring the grease containment. Most manual grease guns can generate 5-10 THOUSAND pounds (psi) of pressure. Much safer to use a grease cup if you have any say in the matter. You just give the cup a turn or two a couple of times a season and the problem is taken care of.
Here's a picture of a grease cup:
Here's another device that has pressure relief:
GUN-filŪ LUBRICATORS : Lube Devices