Before my first sprint tri (in the northern Missouri river in early June - even for a short 500 yds you need a wetsuit) I did a lot of research. I ended up buying one but I did find 1 company that puts the rental price towards purchase if you decide to buy one from them. But I am not much help because now I can't remember the company - out of CA like that helps alot.

So, not much help but to let you know that is an option.

And, yes, wetsuits do make a difference in swim speed and fatigue I discovered
Also for a long distance swim in cold water your body won't be wasting energy keeping you warm