I have 27x10x12 fronts and 27x12x12 on rear vamps on my 01 Kodiak and yes I lost a little power but got is back when the baffle blew out of my muffler. As far as water I would not go any deeper then headlight. I have my bike complete snorkeled because of getting my belt wet all the time.

I would have to say 26 or 27's. You might get rubbing in the front end if you do sharp turns.

I have a yamaha kodiak with vamps. That thing will walk thur anything and rides so smooth on the trails.