I would definitely invest in a quality marine VHF handheld if I were spending a lot of time on the water. And I agree, I would take that over an aviation handheld every time. I expect that SAR aircraft would be listening on marine channel 16, especially if they work near bodies of water.

Many years ago I took a number of flights in small private aircraft, and I seem to remember that one of the radios was always set to 121.5 — I was and am very interested in radios and aviation, and I paid attention to that. Supposedly commercial aircraft maintain a listening watch on that frequency. If the aircraft is in view I’d also expect to be able to communicate with it on a handheld. VHF generally has excellent range when there isn’t vegetation, buildings or terrain in the way, even at low power. Obviously, don’t transmit on aviation frequencies outside of a bona fide emergency or a properly licensed and legal use.

My ham radio handhelds are all programmed for marine VHF. It’s legal to listen to, and in a bona fide emergency it could be legal to transmit. Aviation VHF uses AM where marine VHF uses FM. I don’t remember ever seeing a handheld ham radio that could transmit using AM.