Starting batteries and house batteries do different jobs
A starting battery is built for a short, violent burst of current. Thin plates, plenty of surface area, and it is designed to come straight back up to full charge from the engine. A deep-cycle house battery is the opposite: thick plates, built to give up a small current for hours and then be drawn down and recharged repeatedly. Swap their jobs and you'll kill either one early. Run a starting battery down on the stereo and the livewell all afternoon, and it won't see the end of the season.
Dual-purpose batteries are out there. They're a compromise, though, not a free lunch. They crank okay and they cycle okay, and on a small boat with one battery and a light load that's a fair choice. Got a windlass, an inverter, a fridge or serious electronics? Then you want a separate house bank and its own starting battery, with a switch or an isolator between them, so the house load can't ever leave you sitting there without a start.
A voltmeter reading is not a battery test
Resting voltage only tells you state of charge, and only once the battery's sat off the charger for several hours. 12.6 volts at rest? It can still fail under load. Voltage says nothing about how much current those plates can really deliver. Plenty of batteries look healthy on a multimeter, then drop to eight or nine volts the second the starter engages, and the engine cranks slow or not at all. That is the failure most owners mistake for a bad starter.
A load test is the real measurement. The battery is charged, then asked for a large current for a fixed period while its voltage is watched. Conductance testers do the same job electronically and are quicker on a dock. Whichever you use, charge it all the way up first — a stone-dead battery flunks a load test whatever shape it's really in, and you've learned nothing. Known charge, then test. That's how a bad battery and a charging fault stop looking like the same problem.
The charging circuit: alternator, regulator and voltage drop
A battery that keeps dying often isn't the problem at all. On an outboard the charge comes off a stator and goes through a rectifier or a regulator; a sterndrive's got a belt-driven alternator instead. Rectifier quits? No charge reaches the battery — and some units leak alternating current through as well, which cooks it. So the first thing to check is charging voltage right at the battery terminals with the engine running. Too low, nothing's going in. Too high, the regulator's boiling the electrolyte off.
Voltage drop is the quiet one. Check the charge at the battery, never at the engine; every crimp and terminal and switch in between takes its cut. A corroded switch stud or an undersized cable can read fine at the alternator and still deliver a useless charge to the bank. New alternator? Usually not. More often it's a run that needs cleaning and re-terminating, a tired switch that needs swapping, or a cable that was undersized from day one.
Parasitic draw, switches and terminals
A parasitic draw is anything that pulls current with the key off. A bilge pump float switch. Stereo memory. A chartplotter, an alarm, some accessory that got wired badly. Some of that's fine — your bilge pump has to stay live, after all. The trick is switching everything off, putting a meter on it, and asking one thing: does this draw match what ought to still be connected? A few milliamps is nothing over a weekend. A hundred is a dead battery by Friday.
Battery switches and isolators decide which bank feeds what. They're often wired wrong, or just left in the wrong position. Terminals matter as much. Corrosion, wing nuts where proper hardware belongs, loose clamps: every one of them adds resistance right where the most current flows. Clean lead, the right hardware, protected terminals, cable that's properly supported. Boring work, and it heads off most no-start calls. Fresh water means less corrosion than a boat on the coast fights, but condensation and a damp bilge still do plenty.
Winter storage and sulfation
What kills boat batteries on this lake more than anything? Six months of sitting through a New Hampshire winter. The season's short here. Ice-out usually gets declared somewhere in mid-April and boats come out well before the freeze, so a battery spends more of its year sitting than working. Leave one partly discharged and it sulfates: lead sulfate hardens on the plates, and that capacity is gone for good.
Put it into storage fully charged and keep it that way, on a proper multi-stage maintainer or with a charge every so often through the layup. Cold isn't really the enemy. A fully charged battery stands up to freezing far better than a dead one, since discharged electrolyte is a lot closer to plain water. Disconnect the negative. Keep the top clean. Check the voltage before spring. A battery that reads low in March can be dealt with in March.