Hi Steve,
When the drive train is dialed in properly, I think you can do without the extra hassle of a pump feeding the cooling circuit, if you use a pressurefree system, consisting of an intake behind the prop, angled forward and an outlet out of the propwash under the hull, slanted towards the transom.
This rearward facing outlet creates suction once the system has been filled (by running the boat forward).
Once the air is out of the tubing the flow in the system keeps going, even when you're making the model reverse.
I even use this pressureless system in my fast electrics, where everything sticking out under the hull causes drag, so on those hulls, the inlet and outlet are sanded flush with the hull and still work flawlessly:
Running the tubing inside the hull as low as possible helps getting the flow going, also avoid long lines of silicon tubing, the drag inside is very high.
I use 4 mm thin walled aluminum tubing for the long stretches and only use silicon tubing near the motor (and ESC if required).
Granted, you don't have water coming out of the exhausts, but for that purpose a peristaltic pump would work better than a windscreen washerpump with it's high output (and powerconsumption!).
Also keep in mind the average windscreen washer pump isn't designed to run continuously, the motor is just a simple 540 type, which will overheat quickly when it has to work permanently (don't ask how I found that out...)
I don't recall you mentioning which ESC's you're going to use, but, unless they are extremely efficient and/or much bigger than required, most ESC's tend to run warm/hot when the motor is running partial throttle for a prolonged time.
Adding watercooling may be an option.
Regards, Jan.