Why would anyone want an 8D or even a 4D?? They are so heavy you either have to hire a couple of gorillas and/or a crane or risk crushed disks and pulled muscles to move them, let alone get then n and out of the boat. Since the 8D and 4D are designed as starting batteries for large diesel engines, not sure that they are even real deep cycle batteries. They may be justified for some critical size requirements. Other than that, I'd never consider them as an acceptable boat battery.

Golf Cart batteries are probably cheaper and relatively easily shifted around when the need arises. Since there seems to be golf courses everywhere the sun shines on a regular basis, they are manufactured in huge quantities for economy of production which makes then relatively cheap and available most every where. Four GC batteries wired in series and parallel will give you 400 amps in a relatively compact package or can be split up to fit in the space available.

Nothing wrong with 12v batteries for most boats, in any case. Most boats aren't used for more than day sailing which doesn't take a lot of battery capacity. The problem comes when you start adding electron gobbling ancillaries and unplug from the marina for more than a day. Of course, you can add more 12v batteries in parallel to up the amperage.

You'll still have to charge the batteries on a regular basis whether it's an 8D or a couple of group 27s or GC batteries. If you take the amps out, you've got to put them back.