Installing a set of AGM batteries will reduce maintenance and will increase the longevity out of your battery system. AGM batteries have great recharging capabilities and work well with inverter chargers. I have regularly gotten ten years of performance out of a set of AGM batteries.
I carry a full line of AGM batteries, including Trojan and Fullriver marine batteries. A more cost effective approach may be a set flooded batteries, Trojan T-105 golf cart style 6 volt batteries have been the workhorse for the marine industry for years. With a little care and maintenance they will give a log life and good performance at a good value. Golf Cart style or G-2 batteries work really well with inverter chargers and as a house bank. I have had good luck using Group 31 start batteries for the engine or engines and in AGM applications the Optima spiral wound batteries will start even the largest engines thus saving space and cost.
Maintenance proper installation and charging is key to long life. I recommend having your batteries checked as they get older. Performance starts to drop off as the batteries get older this may be a sign that it is time for replacement. Load testing or digital testing is a good way to see just how your batteries are doing. Keeping the terminals clean is very important to battery life a little dielectric grease on the terminals will help keep the corrosion out and extend life. If you happen let the water get below the tops of the plates fill them with distilled water, charge the battery or batteries and then check them with a hydrometer. If you have bad cell the bulb will not float. A bad cell can take out the whole group and the battery should be replaced. Letting the batteries sit not charged is also hard on batteries and I recommend the same procedure if that happens.
Give me a call I will come down to your boat and check your batteries for you.