You probably want to consult with an attorney there concerning inland salvage rights. For instance, if you salvage a Sea-Doo; the insurance company will most likely abandon the property to you in lieu of paying a salvage fee on the watercraft. Same is true with a boat or other property.
You don't have finders rights to the property, but you should have salvage rights; which means that they will have to pay you for recovering property that they were happy to leave at the bottom of the lake.
Inland salvage is a queer bird, though. I would highly recommend checking with an attorney before undertaking actual recovery operations. Sometimes you are better off contacting the insurance company or owner prior to recovery, as they may abandon claim before you recover the property; which will save you the heartache and stress of having to go through post-recovery negotiations. Be sure to get everything in writing, though.
Tachyon
I am not a lawyer, and I didn't stay at a Holiday Inn Express last night...