You never know about the weather, but I did MFS last June (22nd launch) and was glad to have my rented dry suit from kayakacademy.com. The first couple days were chilly and w/ the high water, long swims were a possibility. We ended up w/ three people in the water first day, all with dry suits, and they were fine. We were all rafters... BTW.
I've since picked up a GMER used on ebay, excellent condition for $550.