In your particular case, gels might be a good solution. With no charging source during the week, their very low self-discharge rate would be a plus. On weekends, when you run the boat, the alternator
will top them off OK.
Gels are indeed sensitive to over-voltage charging, but I doubt seriously if a 5W panel would do any damage to a sizeable gel bank (I assume over 200AH). 5W @ 19VDC is only 0.25A....barely a trickle charge when the sun is shining brightest.
AGMs also have a low self-discharge rate, but really need to have a full charge every couple weeks or they'll sulfate badly.
With no controlled charging source aboard during the off-season, I'd plan to take the batteries off the boat, and store them somewhere you can give them a full charge every few weeks.
Treat 'em right, and gels will last a very long time (witness the two 13-year old golf-cart gels in my basement, which spent their first 10 years on a sailboat. They are still very good.