<font color="blue"> I bought a Gel battery for my boat; expensive, but supposed to be worth it. </font>
Hi, Buck.
Check your charging voltage with that gel cell. Typically, it shouldn't be more than about 14.1 - 14.2 volts. Marine charging systems usually run higher than that. A lot of marine gel cells die from being overcharged.
The battery manufacturer should have published charging specs for your battery. I recommend getting a copy of them, if you don't have one already, and making sure your boat's charging system is compatible with/set up for gel cells.