If you are really serious about getting across rivers, a good dry suit is the way to go. They provide floatation and warmth. A friend who lived on Oyster Bay (Bremerton, WA) used a dry suit to get from his anchored boat to shore. He passed out from drinking too much and washed up on shore a mile down the beach. Woke up in the morning and walked home - warm and dry.
Except that a dry suit is a bit much to pack in a GHB. It would be one thing if I could be certain I could drive to that point, but it's unlikely.