I'm looking to feed water to my whipple's intercooler. I've read bad things about tee-ing into my existing lower water pickup (imco lower) because it drops overall water pressure.

I am running closed loop cooling, so the raw water sees the heat exchanger, p/s cooler, oil cooler, and out the exhaust.

I was going to run a dana marine flush type 3/4" through hull water pickup. The kind you mount from inside and trim flush with the bottom. Will that pick up enough water for the intercooler?

I had the stainless marine water pickup sent to me, but it was way too big for my little Donzi's transom. I sent it back.

Looking for advice on if the through hull flush mount works, or if I need a transom mount pickup, or just T- into the raw water line. Whipple instructions say not to use the raw water pump because it flows water at idle, which is not ideal for an intercooler.

The thru hull pick-ups absolutely work. The Stainless Marine thru hulls are my only water pick-ups. This project is fairly involved and generally will be best accomplished with the engine(s) out. You may want a water cut off valve, etc. This is a good winter project. You will need to drill the hull, mark and cut the pick-up to length and then fill in around the tube maybe with epoxy (can't remember).
At first we ground out a 4-6" channel in front of the pick-up. We ended up filling this in back to flush as it created to much water pressure!
A side benefit of this style pick-up is you may pick up 1-2 MPH if you remove any other pick-up system that protrudes below the running surface. Some step bottoms will also get less air in the water system with this style pick-up.

if you have say 20 to 25 p.s.i of water, then you could plumb it off your raw water supply by y ing the inlet hose before in gets to the crossover.

I'm closed loop cooling- no crossover- raw water doesn't even see the engine, just the heat exchangers and p/s and oil coolers. I'll try the T-fitting first before the through hull.

I agree with Monstaa. If your boat is like most, you should have picked up a significant amount of (like around 10PSI) water pressure with the IMCO lower. I did.
That allowed me plenty of extra water pressure and volume to run my whipple intercooler off the raw water pump. To prevent the intercooler from flowing at idle, I got an adjustable water pressure relief valve from Hardin Marine. It is normally used to prevent excess water pressure in sea strainers. Sorry I don't have the part # handy. I'm sure if you call them, they will find it for you.
Here are some pics. It's the bronze fitting/valve installed right behind the water pump.
Steve

I just put a Whipple on my hp500efi, and Whipple is suggesting going with a transom mounted water pickup to supply the engine, and let the drive supply the intercooler. I added a sea strainer and have lots of water flow. And pressure seems fine too.