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Most deep-cycle batteries are rated in amp-hours (AH) at the 20-hour rate. For example, a "100AH deep-cycle battery" will deliver approximately 5 amps for a 20-hour period before reaching 10.5 volts, at which point it is effectively dead.
As a rule-of-thumb, you should not draw a battery down more than 50% of its rated capacity, thus a 100AH battery really only provides 50AH before reaching 50% depletion. Actually, because of numerous inefficiencies in the way batteries are used -- and recharged -- on boats, it's better to use a more conservative figure, say only 40% of the battery's rated capacity is really usable on a continuing basis.
If you believe that you will need to draw 100 amp-hours from your battery between charges, then you will need 100/.40 or 250AH total capacity. One way to get this kind of capacity rather easily and relatively inexpensively is to use two 6V golf-cart batteries in series. These are rated at about 225AH each, so together in series will give you 225AH total capacity @ 12 volts.
Charging is a very important consideration, too, both in terms of practical solutions and in terms of keeping your batteries healthy. Undercharging is one of the key reasons why batteries fail early, so you'll need some way to replenish the power you take from them efficiently.