You should replace it with a starting battery. Or a "dual purpose" battery.
Starting batteries and deepcycle batteries are manufactured differently.
Both starting batteries and deep cycle batteries are lead-acid batteries that use exactly the same chemistry for their operation. The difference is in the way that the batteries optimize their design:
A starting battery is designed to provide a very large amount of current for a short period of time. This surge of current is needed to turn the engine over during starting. Once the engine starts, the alternator provides all the power that the engine needs, so a starting battery may go through its entire life without ever being drained more than 20 percent of its total capacity. Used in this way, a starting battery can last a number of years. To achieve a large amount of current, a starting battery uses many thin plates in order to increase its surface area.
A deep cycle battery is designed to provide a steady amount of current over a long period of time. A deep cycle battery can provide a surge when needed, but nothing like the surge a starting battery can. A deep cycle battery is also designed to be deeply discharged over and over again (something that would ruin a starting battery very quickly). To accomplish this, a deep cycle battery uses fewer thicker plates.
Does the outboard engine (Force 120HP) emit enough voltage to charge the batteries while running?
I'm not sure how many amps your force produces, but my 94 Mercury stator only makes 16 amps. In comparison a 3.0 Mecruiser alternator makes about 60. Our old outboards were made before small boats had GPS, depth sounders and high powered stereos. I don't know what your typical electrical draw is. I remove my battery after every outing and charge it at home.
One of the members with dual batteries will have to chime in with ideas on the best setup.