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 is the job and a healthy pump does it well. It is not a damage-control system. Once you account for the height it has to lift and the bends in the hose, a typical recreational pump moves a good deal less than its rated flow, and nothing fitted to a runabout will keep ahead of a failed through-hull or a holed hull.
That changes how you should read it. A pump that cycles once after an overnight downpour is doing exactly what it was fitted to do. A pump that runs every twenty minutes through a dry week is not a pump problem at all. It is telling you the boat is taking on water and the pump is the only thing hiding it. We treat a frequently cycling pump as a leak investigation, not as a pump replacement, and the first question is where the water is coming from.
Float switches and the two ways they fail
The automatic function comes from a float switch, either a separate unit mounted beside the pump or one built into the pump body. It fails in two directions and they are not equally obvious. It can fail off, where the float is held down or blocked by a leaf, a wrapper, an oily rag or a build-up of grime in the bilge, and the pump simply never starts. That is the failure that sinks boats on moorings, and it gives no warning whatsoever beforehand.
Or it fails on. The switch sticks closed, the pump runs continuously, flattens the battery and then cooks itself. On a boat left on a mooring that leaves you with a dead battery and a dead pump at the same time, which is the worst of both. Both failures trace back to the same cause: a dirty bilge and a switch mounted where debris collects. Keeping the bilge clean and the switch clear is most of the maintenance this system ever needs.
Testing it properly, not with a finger
Lifting the float by hand proves the switch closes and the motor spins. It proves nothing else. It skips the impeller's ability to actually move water, the hose run, the check valve if one is fitted, the discharge fitting and whether the circuit can carry the current under load. Plenty of pumps pass the finger test on a Saturday morning and then fail to shift anything when the bilge genuinely fills, which is a bad time to learn it.
A real test puts water in the bilge and watches the whole cycle. The switch should trigger at the right level, water should leave the discharge in a solid stream rather than a dribble, the switch should cut out cleanly, and nothing should siphon back in afterwards. We watch the discharge on the outside of the hull while it runs, because a pump that hums with no flow 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 the automatic circuit is wired ahead of the switch, direct to the battery, so shutting everything down when you step off the boat does not shut the pump down with it. A pump wired through the main panel is a pump that is switched off all week on a mooring, and that is precisely the week it is needed. The manual override at the helm is a separate circuit and does not replace it.
Wiring ahead of the switch leaves that circuit unprotected unless it is fused at the source, so the fuse belongs within a short run of the battery terminal and sized to the pump. Connections go above the water line you expect in the bilge, made with marine-grade tinned wire and proper sealed terminals, never twisted and taped. The American Boat and Yacht Council publishes the standards this follows. Corroded connections are the quiet reason a pump that used to run no longer does.
Hoses, through-hulls and where water actually gets in
Water rarely arrives through the hull itself. On a runabout the usual suspects are a leaking transom drain plug or a corroded drain tube, a split in a sterndrive bellows, a deck fitting whose sealant has let go, a rub rail that has pulled away from the hull joint, or a livewell or washdown line that has worked off its barb. Isolating it means drying the bilge completely, then wetting and watching one area at a time until the trail shows itself.
It is worth being blunt about why this matters here. A boat on a mooring through a New Hampshire thunderstorm season has nobody aboard when several inches of rain arrive overnight, and the pump and its float switch are the entire defence. The same plumbing serves livewell and washdown pumps on fishing boats, sharing the same hoses, clamps and barbs and failing in the same ways, so we check them together rather than separately.