I've done the Grand 4 times in July and August. Tried a farmer john wet suit on one trip back in the late 80's. It was ok while I was in my kayak, but as soon as we got out to scout Hance, I quickly overheated. That said one of my buddies had a swim without a wet suit later in the trip. The large, strong eddy (easily about 30 yards in diameter) that he washed into prevented other kayakers from being able to get him ashore, each attempt resulted in another full trip around the eddy. By the time a raft got into the eddy and pulled him into their raft (about 10 minutes) he was quite hypothermic and it took him almost 2 hours in the heat of the sun to feel warm again. Rescue isn't always quick is my point, but on average it takes under 5 minutes. A shorty wetsuit should provide you with ease of mind without causing too much overheating when off the river. The other choice is a drysuit but that involves more money of course. The real answer though is a bomber proof roll in big water and nasty eddy lines.