I once towed a disabled boat a couple of miles into a marina and received a nice bottle of wine as a reward from the grateful owner. The other day, on a sailing forum, someone stated that by simply accepting a tow you entitled the captain of the towing vessel to claim ownership of your boat. Nope. Just because you accept assistance does not mean that you are awarding your boat and contents to your savior.
But, what the rescuer could do, unless you made some other deal at the time in front of witnesses, is make a claim for salvage and let a court decide upon the merit of the claim and make an appropriate award. In order to stand a chance of receiving an award the salvager would need to show that your boat was in peril, he voluntarily provided his assistance, he risked his life or loss of his property in conducting the salvage and, of course, his efforts were successful. Based on that, plus other aspects such as the time he expended and the value of the property he salvaged, the court would make an award.
At sea, sailors are required, by law and custom, to help save lives. They are not, however, required to save property and, when they do, it seems they can seek compensation for their efforts. A nice bottle of wine seemed adequate reward to me and my crew as we settled down to drink it with dinner in a snug anchorage that night.