question about 300ex

hows it going this is my first post, i got a 03 300ex this summer and on the middle of the rear theres a hole that should have a bolt in it [my friends 250 does] he said it needs to be loosened before you adjust the chain my question is since i dont have one there is there a way for water to get into my bearings [i rode at least 3 times a week during the summer and i can move the quad very easy right now with no grinding noise] i just want to be safe so i dont put money into it if i dont have to. thanks ps i go through a lot of puddles here no way to avoid them