When you do replace them, be sure to check out everything MaineSail (a contributor to this forum and others) has to say about batteries.
I can tell you that for most applications, he tends to steer people away from AGM batteries. If you study his information, you'll be in for a few surprises. For most of us, flooded lead acid batteries are the most appropriate. Also, makes great sense to replace 12v batteries with pairs of 6v batteries (wired to produce 12v). For the same footprint, the golf cart batteries store more power, and are heavier duty, allowing more deep discharge cycling and a longer life.
He points out that the 4D batteries are truck batteries, and not a true deep cycle battery. I'll point out that it's a heck of a lot easier picking up and lugging around a golf cart battery than a @#$&ing 4D battery.
The golf cart batteries are a little taller than most 12v batteries, so installations on some boats can be difficult or impossible. They fit my previous boat, a Catalina 309, very nicely. Two 6v batteries have about the same total footprint as a 4D, and on my 309, they fit in the same battery box.
Obviously, if you replace a 12v battery with two 6v's, they'll need to be cabled together. Read MaineSail's info and he'll put you onto a company that will make proper cables for you at a very reasonable price.
You've apparently gotten away with ignoring your batteries (I don't know how long you've had your boat, so pardon the assumption if I'm wrong), but that won't serve you well in the long run. It's really time to replace them.
To me, batteries seem to be equal parts science and mystery, and a pretty dull subject overall. My attitude toward them changed when I studied MaineSail's information, because it gave me a basic understanding of them, and actually made them interesting! Once you grasp the basic concepts and update your power storage appropriately, it will add significantly to your peace of mind. If you do it right, you will likely find yourself showing off your battery installation to your dock mates.
My current boat, a Catalina 28, won't accommodate 6v's without modification, so I simply replaced the batteries the PO had installed with a matched pair of 12v batteries from Sam's Club (batteries were made by Eastern Penn). Not as good as a 6v setup, but good enough for my current needs (PUN!), and they're positioned where I can get to them easily to inspect and monitor their fluid levels.
My apologies to MaineSail for any inaccuracies.
