so after everyone has had their say (including me earlier), no one has REALLY managed to answer the original question about salvage rights.
So I'll do the research:
"When a ship or boat has been rescued or salvaged without prior agreement between the owner and the salvor, the salvor in some circumstances can legally claim recompense or "salvage rights"." -
en.wikipedia.org/wiki/SalvageSo if your kiteboard can be classified as a ship/boat, then I'd say that applies. There's a whole heap more on that site including flotsam/jetsam blah blah.
I think you'd struggle to say a kiteboard is a vessel.