Dual Float Switches for a Boat's Bilge Pump
Here's my thinking



When the bilge starts to fill the upper float is OFF and the lower switch turns ON but nothing happens because the relay switch is open.

When the upper switch operates (lower switch is closed) then 12.7V is fed to the relay turning it ON. As the level of water falls the upper switch opens but there is still a 12V feed from the relay switch through the lower float switch that will keep the relay ON (effectively a latching circuit).

As the level drops the lower switch opens and the relay switch drops out.

Adding a failsafe switch (mechanical override)