[andrew_]

Depending on the vessel, leaks are part of the design. Seals always leak. Salt water is highly corrosive and will always "find a way." I own a smallish boat on the Gulf of Mexico with outboards (lacking the shaft seals that inboard boats have) and water STILL finds a way into the bilge. I have three pumps, one main and two redundant, just in the event one of my scupper seals goes, something/someone strikes my boat hard enough to puncture the hull below the water line, or one of my through-hull fittings (like raw water pump) develops a leak, while the boat is out in the Gulf or sitting at the marina.

Might seem like overkill where regular maintenance should be sufficient but you can never have too many redundancies to keep a boat afloat. Murphy's Law very much applies.