I'm with seadaddler - for about the price of ONE trojan 8D gel you can get six 6v golf cart batteries. What's an 8D? something like 225AH? 6 GC2s would be something like 670 AH. - IOW, your bank capacity would be the same with 6 6v batteries as with your 3 8Ds. But it's around a third of the price. A normal human can lift them in and out of the boat. You can buy them anywhere.
Of course flooded batteries have drawbacks too; including they're a bit taller than an 8D, you need to water them, they don't last long upside down, and your charger needs to have the right charge profile for them.