Another reasonable option would have been to call him and say "I heard you lost a prop recently. I found one that might be yours from what I heard." He is not the only one to lose a prop and if the boat is underway, finding it is problematic. Offering it too him and negotiating a fair fee would have been reasonable since you did it completely on spec, in really crappy conditions If you had come up empty, this guy would never had known and if you sold the prop, this guy would never have known. He went out and bought a replacement, so I think that would classify it as abandoned and fair game (I would not have sold it without offering it to him first).
If you did it for fun and you returned it to him with no expectation of reward shows you have a bit of class. When I have been asked to look for something, I get some pre-negotiated compensation. This was a fun task, if you had offered the return of the prop for compensation for your time and effort, I doubt he would have complained.