I seem to remember the salvage thing came up several years ago, and I think the end consensus was there are no salvage rights in fresh water.

You salvage someone’s boat without their permission and you are in danger of being cited for theft, the people have no obligation to pay you for your efforts and even worse could sue you for any damage that might have been caused during the salvage attempt.

Sounds like SuperFly did the right thing in getting the owner’s permission and working out some sort of agreement on payment beforehand. Probably not a bad idea to get something in writing, with a disclaimer absolving you from liability from any damage claims next time.

I also know that at least on the Deschutes in Oregon, if you sink you're boat you are responsible for getting it out. Either you get it organized or the BLM will and then charge you for it.