Wetsuit or rash guard?

I'm going to do a swim race at Lake Pend Oreille in early August. I will be in the water for about an hour and the water temperature in there is anywhere from 55 - 70 degrees from what I have heard. I have never swam this long in water this cold before and was wondering if a rash guard would at all be suitable to keep me warm in those conditions or not? I just don't want to shell out $70+ if I don't have to!