Boats take a pounding as do farm tractors and OTR semis. Heavy duty batteries are usually required for such instances. Sulphate scale develops on the plates of batteries due to the electrical process of using and recharging and all. There are battery chargers designed to negate this buildup...pricy. The sulphate scale can be knocked off the plates and can pile up at the bottom of the battery, especially on densely packed batteries...lots of amp hours on a small case types where the plates are long and bridge across the cells allowing the charge to bleed off. If you have a white plastic case, you can put a flashlight on the other side of the battery and see the plate length.
The other problem is when the scale develops and doesn't fall off causing a reduction of the exposed plate surface for current carrying capacity....which doesn't seem to be your problem but it's a problem with batteries. A load test is where you put a resistive carbon pile across the battery and measure terminal voltage while pumping out a couple hundred amps. If the voltage falls fast and more than a couple of volts you are sulphated up which is easy to accomplish when using a deep cycle battery for engine starting....which you said you don't. In that case there is less surface area as plates are thicker so it's easier to coat them and reduce current capacity. Most service facilities have load testers that test terminal voltage with starting current amperage applied. I have one I bought from Harbor Freight years ago, about 50 bucks. Works great. Just clamp it on, get a voltage reading, turn the knob to decrease the resistance of the pile till you get to 200 amps (my number) and check the voltage. I like to have at least 11v of terminal voltage on a fully charged, stabilized battery at 200 amps to consider it good. Get the data and get the pile off the battery as it gets hot quick.
I'm not a battery designer, producer, nor seller. Just what I do with mine and it works for me.