Hmmm. OK, every mechanical switch has a way to fail. Broad statement, but as the screen/filter gets finer it also has a greater chance of clogging, and sand can get through in any case. Pivots and rods in cases can stick.
There are two problems here. A mechanical switch can stick in the open or the closed position. If in the open position, water accumulates, potentially sinking the boat. If it sticks in the on position, it can kill the battery
and then allow water to accumulate and potentially sink the boat. In either case you don't have a bilge pump
. I've had both cases; I've got one right now in that my engine
room pump is not coming on as small amounts of rain water come down the ventilation. I damned near sank our small open boat a few weeks ago when the pump stuck on, and rain filled the boat after the battery
Electronic switches must have ways of sticking, but they are not mechanical. The water Witch won't operate if we fill the engine
room with oil, and we did that once by failing to open the fuel
return from our main diesel
. It popped a hose and put about 100 gallons of diesel
in the bilge. That didn't endanger the boat, because the oil just moved from the tanks
in the engine room to the engine room itself, but it was one honking big mess to clean up.
Overall, I'm going with electronic (water witch), plus a spare float switch tied to a high water alarm