A must have when owning an Apex...

[Engloid]

Any of them with two wires like this will work. It's essentially a board with two wires on it. When water or moisture connects the two together, it sends the low voltage signal back to the BOB and it will read as "closed." You can then set an alarm to go off when that switch is closed.
You don't even need the small circuit board that comes with it. I can't remember what it's for.

https://www.amazon.com/uxcell-Raind...+uxcell&qid=1549816464&s=gateway&sr=8-2-fkmr0

 

[JDowns]

Breakout box.

I did a version that uses 3.5mm jacks and panel mount jack for less than $12.

Assembled.