As Grunt says - the pin should be oil and watertight anyway. If it's leaking, then you can replace the seal (not a small job, but far from Rocket science) to improve the 'watertightness'.
If you mean the ball though - that's a different matter - there are gaiters you can buy, how watertight they are I've no idea. Bearing in mind they're clipped in place with, effectively, cable ties I'd suspect they're not watertight.
Cheers,