They cannot handle the repeated deep discharges typical of marine use or the constant pounding they receive at sea. Given all this you will need a 400-Ah battery to meet your 100 Ah daily energy habit. In general, a battery rated at four times your daily usage will be adequate. Engine Starting Batteries Do yourself a favor by reserving an adequately-sized, fully charged battery solely dedicated to starting your engine. If you don't replace what you use, your batteries will eventually go dead, no matter how much battery capacity you have on your boat.
Most boats charge their batteries with an engine-driven alternator.
If your engine running time is minimal, you want to charge as quickly as possible, without damaging the battery. Battery damage begins when the internal temperature becomes too high, causing it to gas and heat up. If it feels warm to the touch, it's getting too hot. Use a voltage regulator to taper off the charge to prevent this. Typical automotive alternators can't run continuously at full load in the warm temperatures found around boat engines, or in warm climates.