Jim is spot-on with deadrise-- it's the vee in the bottom of the boat. The less deadrise you have, the easier the boat is to plane. It also requires less horsepower to run and is more stable. However, the less deadrise, the more likely the boat is to pound when running into or with a chop. The more deadrise, the smoother the rise, the more horsepower required to run the boat, etc.
Boats are big trade-offs-- you need to identify what you want in a boat and determine those factors that provide for your needs.
A fuel-water separator is an excellent idea on any boat-- Basically, a separator is simply an in-line filter that is installed between your fuel tank and the engine(s). You change the filter cartridge seasonally (depending upon use). You should be able to buy one for less than $50; I have yet to buy a cartridge for mine, so I can't tell you how much they cost, but it should be inexpensive...