I'm in a similar situation to the OP. I'm going to Hawaii in May. I currently have a Canon A720 with a good underwater housing, but I've been disappointed with the performance. Shots often come out grainy and/or blurry. Curiously, it takes pretty nice underwater video, though. I have a Nikon D90, but no housing. The cheapest good housing for that is around $1400 I believe, little too steep for me given how infrequently I'd use it. I know there are cheaper options for the D90, that are basically watertight bags with a glass lens opening. I've read somewhat bad reviews for these, in that taking photos is difficult, but not that they leak.
I'm debating on getting one of these cheap housings for my D90, or getting a better P&S that is either waterproof or has an optional housing for underwater.