I wonder if you need watercooling.

I use a Torpedo 850 with a 12v 3AH lead acid battery without any cooling at all on my 34" Aerokits Crash Tender.

I get five to ten minute run time on full power, after which time the motor and battery are stone cold.

Goes like the clappers at top speed, and goes for hours when just ticking over.

Hiya
GEE DEE models in Nottingham do a cooling coil for large motors - look at their website or ring and ask for Paul, their expert in RC electric boats
Cheers
Mike

Firstly "welcome to the site" and hope you enjoy your visits here as many others do.

To go a little further about the motor cooling, if you are going to run a cooling pipe around the motor, Copper is the best you can use, as the "metal" conducts heat so easily and quickly, wheras silicone piping would only pass water and nothing else so to speak.
If you are looking for a cheap source of copper pipe, suitable for bending, go to your local car spares shop and buy some "replacement brake pipe" this is copper based and is bent easily around a suitable former [or dowel rod/tube]. Please try to ensure the tube/s when bent are a tight fit around your motor, it will not serve any purpose if its a loose/floppy fit, nor will it do any cooling.
you can also run a cooler pipe under the heatsink of the ESC if you wish, to aid cooling to that also, make it part of the same circuit and the less water pipes and joins inside the hull, the safer it will be from bursts or hoses popping apart, especially when in the middle of the water !!!!!

best of luck .