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. Using one for the other job shortens its life fast. A starting battery run down to power a stereo and a livewell all afternoon will not last a season.
Dual-purpose batteries exist and they are a genuine compromise, not a free lunch. They crank adequately and cycle adequately, and they are a reasonable choice on a small boat with one battery and a modest load. On anything with a windlass, an inverter, a fridge or serious electronics, the right answer is a separate house bank and a separate starting battery, kept apart by a switch or an isolator so the house load can never leave you unable to start the engine.
A voltmeter reading is not a battery test
Resting voltage tells you state of charge, and only after the battery has sat off charge for several hours. A battery reading 12.6 volts at rest can still fail under load, because voltage says nothing about how much current the plates can actually deliver. Plenty of batteries that look healthy on a multimeter drop to eight or nine volts the moment the starter engages, and the engine then cranks slowly 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. Either way the test happens with the battery at full charge, because a flat battery fails a load test regardless of its actual condition. Testing at a known state of charge is what separates a dead battery from a charging fault.
The charging circuit: alternator, regulator and voltage drop
A battery that keeps going flat is often not the problem. Outboards charge through a stator and a rectifier or regulator; sterndrives use a belt-driven alternator. A failed rectifier stops charge reaching the battery, and on some units it also lets alternating current through, which cooks the battery. Measuring charging voltage at the battery terminals with the engine running is the first check: too low and nothing is going in, too high and the regulator is boiling the electrolyte away.
Voltage drop is the quiet one. Charge is measured at the battery, not at the engine, because every crimp, terminal and battery switch in between costs voltage. A corroded switch stud or an undersized cable can read fine at the alternator and still deliver a useless charge to the bank. The fix is usually not a new alternator. It is cleaning and re-terminating the run, replacing a tired switch, or correcting a cable that was undersized when the boat was built.
Parasitic draw, switches and terminals
A parasitic draw is anything pulling current with the key off: a bilge pump float switch, a stereo memory, a chartplotter, an alarm, a badly wired accessory. Some of that is normal and necessary, because the bilge pump has to stay live. The test is to measure the current with everything switched off and decide whether the draw matches what should 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, and they are often wired badly or left in the wrong position. Terminals matter as much. Corrosion, wing nuts instead of proper hardware, and loose clamps all add resistance exactly where the highest current flows. Clean lead, the correct hardware, protected terminals and properly supported cable is unglamorous work that prevents most no-start calls. Fresh water means less corrosion than a coastal boat fights, but condensation and a damp bilge still do plenty.
Winter storage and sulphation
Six months sitting through a New Hampshire winter is the single biggest killer of boat batteries on this lake. The season here is short, ice-out is typically declared somewhere in mid-April, and boats come off the water well before the freeze, so a battery spends more of the year not being used than being used. A battery left partly discharged sulphates: lead sulphate hardens on the plates, and the capacity lost that way does not come back.
The battery should go into storage fully charged and stay that way, either on a proper multi-stage maintainer or by being charged periodically through the layup. Cold itself is not the enemy; a fully charged battery is far more resistant to freezing than a flat one, because a discharged electrolyte is much closer to water. Disconnect the negative, keep the top clean, and check voltage before spring. A battery that reads low in March can be dealt with in March.