Go with the TT bike- it was made for this kind of race
as far as water goes
I have heard that wearing a camel back under your jersey can actually be more aero than not having one (in fact people with a natural hump in their back ala Lance Armstrong, get this advantage without one). I've even heard that the UCI has considered banning them in TT's because of this (no worries, tri's have completely different governance and rules!)