Which is the overboard drain for the ice box sump. I've never had a problem with back siphoning into the ice box so I guess it must be well above the WL, even at extreme heel. You'd want to know that for certain.

My electric Rule bilge pump has a check valve in the long line from the deep sump to above WL at the stern. It's less than perfect and a long way out, but's it's the best I can configure. A great improvement was to place the pump and float switch about 5 gallons above the bottom of the sump(it's a big deep sump). That has eliminated most of the cycling that can occur on my mooring(my old boat takes some rain water through the wooden cockpit hatches).

The manual pump drains the lower 5 gallons and lets me keep track of the level by the # of pumps with the manual. 20 strokes of the Gusher pump means the switch has been activated at least once.