John:
On of the best of the Lithium Iron Phosphate (LiFePO4) batteries for marine (and aviation) use is the RELiON, and their Web site offers a wealth of info, prices, and comparative charts if you browse it fully: www.relionbattery.com
Their RB300-HP, for example, is a 12V 300Ah dual purpose starting/house battery that offers at least 13,000 cycles, measures only 20.5” by 10.5” x 9” and weighs only 82.7 pounds. Due to its long duty life, it ends up costing only 1/3 as much as an AGM battery and charges 4 times faster. Here is a useful comparative chart:
