It really depends on terrain and distances involved. Running 5 watts into a rubber duck antenna will not get you very far unless you're on wide open water. Where I boat, surrounded by hills & small mountains, my handheld is virtually worthless as an emergency radio. I suppose it could be useful when approaching a gas dock to see if they're open and that kind of thing.
I struggle with the same thing. A fixed radio with real antenna is the best choice, but a real antenna on a bow rider, especially one less than 26', looks silly.
Shakespeare does make some small SS antennas
(one example) that would be much less obtrusive, and which would work far better than a handheld. If you decide to do that, be aware that radiating 25W from an antenna right next to a person's head is not a very good idea. It would be fine for receiving and for the occasional short transmission, but if you see yourself rag chewing on a fixed mount radio, move your passengers away from the antenna.
Another compromise would be to mount an antenna like the one mentioned, and connect your handheld to it (you'd obviously need a radio with a detachable antenna). You'd still only be generating 5W output, but 5W into a real antenna is far better than 5W into a rubber duck.