that one is the classic civilian distress freq for aviation. Trouble is, exceedingly few non-airline planes are monitoring that freq. And, I'm not even sure airlines do great job monitoring that. I never did on all my overwater flights. Far more used are the FSS freqs and the unicom freqs for uncontrolled airports such as 122.7 and .8. In "busy" island space, you are on some approach freq; in the middle of nowhere in uncontrolled space, you are busy listening to lots of planes for a dozen miles around at multiple airports. Best to have a green book with the listings of the freqs being possibly used along with that airband radio.
I have an Icom A110 that I have considered putting in my next boat. But not really for emergencies.
Diver,
You're right to a point but 121.5 is still the frequency. Just recently no longer monitored from satellites but still WAY more monitored from airline and GA. And no one constantly monitors unicom, it would drive you nuts! I still monitor it in my plane, (and did as SOP in the airlines). And hear all kinds of conversation at times.
As for using it in a boat, I'd be surprised if you didn't get a response from someone.