You'll need to use a water proof switch and mount it where you have access to it yet won't be getting kicked while getting on and off the boat.
Look at te wiring diagram for the trailer switch and wire this one in parallel. You won't need to run wires to the control just to the up soloniod at the pump and bypass te trim limit switch. I can draw something up but not till next week if you want.
1997 2400 SCR
5.7 Vortec / Bravo 2