you're not going to like this..... but 99% of the time I would never consider having a weapon with me on the water.
I work on saltwater (some years I'm in my small 17' skiff more than I'm out of it) and I wouldn't even consider carrying a sidearm or long gun with me unless it was for a specific situation -and only then.... If you work around saltwater it's tough enough keeping stuff meant to be there from corroding or rusting - much less a decent firearm. I routinely handle sharks up to about ten feet long with only gloves on my hands (among other hazards in the saltwater portions of the Everglades) and have never needed a weapon on the water in over forty years. Recently I've considered adding something to kill pythons with (yep, we've got both Burmese and Rock pythons in numbers where I work.....) but still haven't made any decision. Since I'm a permitted guide in Everglades National Park, firearms and keeping your license might just be a problem...
Funny thing, back in my police days, once or twice I actually did use one of my skiffs for special situations with one or two SRT members aboard. I had them pose as fishermen until we could get close enough to the water side of a marina where they removed their foul weather gear and stepped up on the dock in uniform with MP5's in hand to deal with a problem child...
While I've never carried a weapon on the salt... it's an entirely different situation when I'm towing my skiff from one place ot another and I always have a weapon somewhere nearby in my vehicle. South Florida can be an adventure day or night and I'm towing an average of 20,000 miles every year - much of it either before sunup or late, late at night...