The water bug sensor measures conductance, or impedance, I'm not sure which, and doesn't have a mechanical OPEN or CLOSED switch, so it must be run to the watebug control unit. The control unit provides an OPEN or CLOSED output signal that can trigger the Apex, so no, you cannot run the waterbug senor directly to the Apex or Apex BB.
There is no way to expand the number of discrete inputs from one Apex base module. You have to buy an expansion module if you want more than 6 discrete inputs. However, while I haven't tried wiring multiple switches to one input in parallel, I did consider it as an alarm system for leaving doors open on my stand and I think it would work. If you do wire multiple float or other mechanical switches to one input of the BB or Apex, you would not be able to determine which of the switches activated.