If you get a good GSP with solid drive, he'll go in cold water.

I have a 7 month old pudelpointer. I put a couple of pictures of him here (scroll down):

http://www.huntingnet.com/forum/tm.aspx?m=1776547

As you can see, he's pretty thin on his underside. Despite that, he's typically in the water going after a dead duck as soon as it hits the water. I would say the water is in the low 40s/high 30s right now (not sure where that falls on the cold/cool scale). While he's in the water, he's not cold, but as soon as he gets out, he does get cold. He was shaking a lot, and even chattering his teeth. I don't use a vest out of fear that he'll get tangled up in something (more due to where I hunt). What I have found that helps tremendously is bringing an old wool jacket or blanket with me. As soon as he's out of the water, I wrap him in the wool for about 5 minutes. After that, he's fine. No shaking; no teeth chattering.

Good luck.