Just buy as good as you can afford. NO, price does not GAURANTEE waterproofness, but the warranties are better if you do get a bad pair.
Having said that, my only other suggestion is to go with a pair that has leather bottoms. They ALL begin to leak a little after some time (standing in water). You can get by a few years more years by snow sealing the seam where the bottom meets the sole if you go with leather.
I got 7 years out of mine in the swamps of Louisiana. I wear them all year for snakes in the warmer times and warmth in the colder times. For just crossing water they should be waterproof for a little longer IF you take care of them. I have some Chippewas right now but I am looking at the Danner Pronghorns as a replacement this year (18" of protection from snakes AND water depths).
Just my thoughts, but I can't imagine a worst environment to try them out.