Paul wants to have both a bait system and raw water washdown using the same
pump-he says:"..The pump I am planning on using is a Johnson 5 gpm model.
It is a diaphragm type with a pressure switch built in to it so that when I
operate the trigger hose assy. (supplied) the pressure drop activates the
pump. ....."

Paul most of the boats I have had (or seen) use a flexible (rotary) impeller
pump (similar to Rule or A****er bilgepumps 500gph to 750 gph) for the bait
system, with the diaphragm type pump usually used for a freshwater
galley/shower)system.
The rotary baitpump is a continous duty item that is not self-priming but has a
low amp draw, is generally trouble free and inexpensive to replace when the
time comes. With a "Y" valve the larger gph units can double as a washdown
pump.
I would use a through hull (bottom) pickup (careful to install so it won't hit
the trailer bunks!) with a good seacock, not just a hardware store ballcock,
mounted just inboard.
Use 100% stainless, band AND screw hoseclamps. Check, just 'cause it says
"stainless" doesn't mean the screw is. Double clamp everything you can as most
of this will be below the water line!
Check with what works for other folks in your area, and West Marine or BoatUS
have usefull info in their catalogs which you can access online.
Good luck
Mike