If you have expended measurable effort in the guarding and protection of the vessel from further harm (IE you found it adrift, caught it, secured it safe from drifting to further harm, took care to cradle it safely and properly as the water receded, etc. ) and by your actions kept it from further dmage or destruction, you have a maritime salvage claim against the value of the vessel, and the insurance company will negotiate compensation with you, as you will have minimized their loss.
In other words, if you see a chance to help somebody's boat, no it doesn't become your boat, but you're more than doing the right thing if you can save the boat, and if you save a few different boats there will be enough compensation in it for you to buy your own boat.
So get out there and save some boats. Happy boat owners also tend to reward those who save their beloved boats since insurance compnay depreciation on a loss really sucks.