Purchase a model tunnel hull boat, eg: TS2 or Bandit or a H&M: Purchase a .21 powerhead engine:or a complete outboard from O.S. Engines or Thunder Tiger. You will need a leg if you just go with a power head or electric motor.
OR, make it electric with a big 600+ size outrunner brushless motor.
Get a 2 or 3 channel radio from your hobby shop.
Get one of the best if not the best props from props4u. Seek help and get it from intlwaters forum.
Vid's of electric tunnels can be found on you tube if you search for "quick H&M" or "tunnel"
The mighty CMB/Gadjet piped TS2.
Beautiful props from Props4U.
The H&M Skater, 600 electric/5s lipo.
You will need a mount too - ebay.