Last fall I purchased 3 new NAPA Gel Batteries for my boat. I had them hooked up to my onboard charger most of the winter. This spring they don't seem to hold a charge like I think they should. My question is : Can you charge gel batteries with your onboard charger that worked with wet batteries ? Everything I read you need a special charger for gel batteries.
Any Thoughts ?

which do u have ?
agm is Absorbed Glass Material
gell is basically a standard wet style battery design with a gell substance used instead
and yes they take a different charger than a wet battery
some newer on board chargers are smart design and charge accordingly to battery style,wet/gell/agm
so gotta see what charger u have name and model to look up its charging mode
get them load tested as if are weak use the warrenty and exchange for new,,,,
as under 1 yr should be free if bad

Need a compatible charger, the smarter the better.

OK , here's a dumb question .
If they require a special charger how do they manage when hooked up to any 'ol engine's alternator ?
btw/ I once had a gel battery that I got about 16 years out of .

Because long term trickle or pulse charging is quite different than constant electrical load and use in a vessel. Its the longer term charging storage where the AGM need a little different attention.

Personally I use a CTEK battery charger. They will do motorcycle, AGM or regular batteries. The device is small and will load test, and charge through various cycles dependent on battery type. They also will not constantly feed voltage if the battery doesn't need it.