Generally people are interested in how to prolong the life of their batteries. Much of the answer is to avoid several practices that kill batteries quickly. If you are new to boating, this is where you should start.
1) Ignore the battery. Do not monitor the electrolyte level if you have a "wet" battery. If you do not know what you have, you should assume it is the "wet" variety until proven otherwise. The battery will completely dry out in anything from 6 months to 3 years ands you will be shocked, shocked to learn you have no power at the start of your annual vacation. Also shocked to learn that batteries require a small amount of preventative maintenance. Modern automobiles contain "maintenance free" batteries so generations have grown up not knowing that the most basic (least expensive) batteries consume water (WHICH MUST BE REPLACED) and must be monitored every three to six months.
2) Do not keep your batteries charged. Have no battery charger aboard your boat and no shore power cord. Do not even connect a small solar photovoltaic panel to keep the battery topped-up. You can also leave the battery charger turned off for long periods of time. Batteries "self-discharge" as rapidly as 10% per month. In a partially discharged state, the sulfate crystalline material in the battery plates gradually changes form. The sulfate goes from a readily soluble form that can be returned to the electrolyte when the battery is charged to an insoluble form that is essentially locked up in the plates forever. The effect is that the battery operates without all of it's chemicals available and it becomes electrically smaller. If it started out as a 100 ampere-hour battery it can rapidly progress to a 30 ampere-hour battery that does not provide as much starting current and does not keep the lights burning as long. The end state is a battery that will not take a charge at all.
This process is referred to as "sulfation." Your know-it-all neighbor at the marina might even tell you so when your boat will not start in spite of having charged for several days. Batteries begin sulfating almost immediately when you delay recharging them but it usually takes a few days to a few weeks for the lead sulfate crystalline structure to get to the extremely resistant state. Why does the crystalline structure change? Lead sulfate can assume many different crystalline forms. As initially deposited in the plates, the crystal form is weak and contains a relatively high amount of energy. As they sit, the crystals want to assume the lowest energy state possible. Lead sulfate crystals transform all by themselves from whatever they are to the another form that has less energy. This takes time at "room temperature." But inexorably they transform from state to state until they are in a crystalline form that cannot be reversed by simply recharging batteries as we normally do.
3) Use an inappropriate battery charger. Charger technology has evolved over the past 50 years, particularly as better and more complex semiconductors have been developed. The charger that came with your boat may have been the best that was available "back then" but it may treat your batteries badly enough to kill them in a year. This is particularly true if you have changed to one of the new Gel or AGM batteries in the interest of getting better battery life. The old technologies can be managed satisfactorily if you visit your boat every few days, but few of us can spare the time to do that. The most common problem is "overcharging," which drives the water out of the batteries at an accelerated rate. If it's not that it is "undercharging" and the batteries never get fully charged and slowly sulfate. By "charger" I include the alternators on your engines. If you are going to switch to one of the new, long-lasting, maintenance free battery types you should have your system checked over by a professional electrician. Issues that did not become significant when you replaced your batteries every two years can rise to the forefront when you are trying to get five to seven years out of a set of batteries.
4) Connect batteries of different sizes and ages into a big battery bank. While not strictly true, the rule of thumb is that a mixed bank of batteries will perform only as well as the worst battery in the bunch. While this may not kill your new battery immediately, it will shorten it's life through the mechanisms previously discussed. If you want a big battery bank, seek professional help. There are enough rules about batteries and battery banks to fill books. People have written them, but none are perfectly complete. None give you a trained set of eyes to spot problems that may not be obvious.