I have cut down plastic barrel to fit under drive. had problems with muffs not getting enough water to pump. normally use 2/3 of barrel and put thru hull fitting in bottom for hose fill/drain.

a single stage water pump will use all the water a garden hose will give at idle speed,a 2 stage pump is not going to get enough water on the hose even at idle.2 impellers =twice the water flow .

I take the outflow hose from the intercooler pump and wye it into the incoming water supply hose going to the sea strainer. As is probably your case, both pumps draw from the sea strainer. Easy for me as everything is AN plumbed. There is no 'wasted water' except what the pump cannot draw is dumped out of the through hull intake and since the system is not closed, you do not have to worry about too much or too little (during a quick rev of the engine) flow from the water supply. Yes, the sea strainer has a fitting welded in for the fresh water hose.

2 or 3 garden hoses is what I've seen, and or restrict but not block the intercooler stage outlet. Water can't get out only so much can come in.

Yes, SFOcean. I was thinking about something like that as well. Just couldn't figure it out as yet. In my case the intercooler is braided hose and the engine is rubber. But, for sure there has to be a way.

to clarify, the garden hose is plumbed into the strainer, it is capped air tight when not in use.