Straight lake water from the intercooler strainer; fyi, Hardin's is 60 micron, the screen in most nozzles is 80 to 100.
You will need to clean out the hard water deposits once in a while though.

Alky Injction is out of Tampa Florida...
He is a big name in Corvettes for the meth stuff... I am sure he would be intrested in making some Boat kits...

Just some food for thought.. At about 1400 hp on 91oct with 18psi on 9 to 1 I was burning about 12gph of 50/50 water meth. The meth output is proportional with boost on a good kit. Doing some quick math, you will probably go through a lot of methanol.

Earlier post was correct, blue washer fluid is about 35% so you will use that much more.. If you were creative you would have a tank for straight meth with its own pump and nozzle and a pump and nozzle for the water thereby making the mix with nozzle sizing. That would cut cost and the tank size down a lot.
Just remember the weight of water is different from water when sizing your nozzles.