What a bilge pump is for, and what it is not for
A bilge pump is built for nuisance water. Rain, spray over the bow, a little seepage past a shaft seal, meltwater off a cover: that's its job, and a healthy pump does it well. Damage control is a different thing. By the time you count the height it has to lift and the bends in the hose, your typical recreational pump moves a good deal less than its rated flow. Nothing you'd fit to a runabout will keep up with a failed through-hull or a holed hull.
So read it accordingly. A pump that kicks on once after an overnight downpour? It's doing exactly what it was fitted for. One that runs every twenty minutes through a dry week isn't a pump problem at all. The boat's taking on water, and the pump is the only thing covering for it. We treat a pump that cycles a lot as a leak investigation, not a pump replacement, and the first question is where that water's coming from.
Float switches and the two ways they fail
The automatic part comes from a float switch, either its own unit mounted beside the pump or one built into the pump body. It can fail two ways, and one's a lot sneakier than the other. Fail off: the float's held down or blocked by a leaf, a wrapper, an oily rag, or plain grime built up in the bilge, and the pump just never starts. That's the one that sinks boats on moorings. You get no warning at all.
Or it fails on. The switch sticks closed, the pump runs nonstop, runs the battery dead and then cooks itself. On a mooring that's a double hit: stone-dead battery, dead pump, both at once. And it's the same culprit as before. A dirty bilge, with the switch sitting right where the junk collects. Keep the bilge clean and the switch clear. That's most of the maintenance this system will ever need.
Testing it properly, not with a finger
Lifting the float with your finger proves the switch closes and the motor spins. That's all it proves. It tells you nothing about whether the impeller can really move water, or about the hose run, the check valve if there is one, the discharge fitting, or whether the circuit carries the current under load. Lots of pumps pass the finger test on a Saturday morning, then can't shift a thing when the bilge truly fills. Bad time to find out.
A real test puts water in the bilge and watches the whole cycle. Does the switch trip at the level it should? Does the discharge throw a solid stream, or just dribble? Does it shut off clean, with nothing siphoning back in after? We watch the outlet from outside the hull while it runs — a pump humming away with nothing coming out is far more often a blocked, kinked or collapsed hose than a dead motor.
Wiring, the battery switch and fusing
An automatic bilge pump should still work with the battery switch off. That means wiring the automatic circuit ahead of the switch, straight to the battery, so shutting everything down when you step off the boat doesn't shut the pump down with it. Wire it through the main panel and it's switched off all week on the mooring. Which is exactly the week you need it. The manual override at the helm is a separate circuit, and it doesn't replace this one.
Wired ahead of the switch, that circuit has no protection unless it's fused at the source, so the fuse goes within a short run of the battery terminal, sized to the pump. Connections sit above the waterline you'd expect in the bilge, done with marine-grade tinned wire and proper sealed terminals. Never twisted and taped. The American Boat and Yacht Council publishes the standards this follows. And corroded connections? They're the quiet reason a pump that used to run doesn't anymore.
Hoses, through-hulls and where water actually gets in
Water rarely comes through the hull itself. On a runabout, round up the usual suspects: a leaky transom drain plug or a corroded drain tube, a split sterndrive bellows, a deck fitting with its sealant gone, a rub rail working loose from the hull joint, a livewell or washdown line that's crept off its barb. To find it, dry the bilge out completely. Then wet one area at a time and watch, until the trail shows itself.
Why does it matter so much here? Picture a boat on a mooring in a New Hampshire thunderstorm season. Several inches of rain come down overnight, nobody's aboard, and that pump and its float switch are all she's got. Fishing boats run their livewell and washdown pumps off the same kind of plumbing — same hoses, same clamps and barbs, same ways of failing — so those get checked in the same visit, not one at a time.