Please understand that the following comments are a generalization and may vary from one battery manufacturer to another. Your individual battery manufacturer specifications should be followed.
Open circuit voltage of typical lead acid batteries is 2.1volts/cell or 12.6 volts for a 12V battery. End cell voltage for wet cells is 1.75 volts/cell or 10.5V under load. This is the lowest voltage a battery can typically go before damage and usually also represents the 50% discharge point of most batteries. So your level of discharging your batteries may not represent a major removal of total capacity.
However, the battery voltage to capacity discharge curve under load is not linear. One way to know for sure is to monitor the amp/hours in/out of the battery. Charging the batteries to the float voltage level (typically 13.3V or 2.22 volts/cell) will only replace 80% of the charge. The remaining 20% capacity takes time (about 80% of the time) to fully charge the battery. Some chargers boost the charge voltage to try and minimize this charge time (the three stage charger mentioned previously) but this is NOT recommended for sealed batteries. This boost voltage also increases the water loss of a wet cell and increases the chance of particulate precipitate, a killer of batteries. A good battery monitor, not just a voltmeter, would be a worthwhile addition to your powerboard. Insufficient charge time, leaving your batteries not fully charged, will also kill your batteries.
Another way is with a hydrometer check for each cell as suggested by camaraderie. Be aware the reading will vary depending on the temperature of the batteries, so warmer batteries will have a lower specific gravity reading at full charge then colder batteries.
The open circuit cell voltage method for checking batter capacity is linear but requires that the batteries are without any load for an extended time, ˝ hour seems correct. The open cell voltage will also be dependent on battery temperature and can vary over 0.1 points of specific gravity from a hot to cold battery (100°F to 15°F). An open circuit 12V battery voltage of 12.65V is typical for a fully charged battery and 12.24V for a 50% discharged battery.
It all depends on how much reserve time you want on you boat before recharging. Do you have enough? Only you can decide. Your first step should be doing a full energy audit on your system so you know how may amp/hours watt/hours you are using so then you can decide how much battery you need.