I agree with Roadie Gal- the longer the swim the more I want my wetsuit. It's my "security blanket" when I'm in the water a long time (and anything that helps my legs stay bouyant without me kicking is a good thing).
There's a place called www.wetsuitrental.com A friend used them for a tri earlier in the year and they were wonderful. He called them and talked to someone who helped him decide what size he'd need and then they sent him three choices to wear. He kept one, sent the others back- then wound up buying one later in the summer.
If you want one, I'd rent one from someone to see if you like it before you drop a couple hundred clams on a new/untested one. IMO

I'd loan you mine, but it would only come to your knees.