Don't they make stupidly powerful inboards then...? 
Yes. Sterling Performance in Milford, Michigan is considered to be one of the finest marine engine builders in the world. Several of the top teams in off-shore powerboat racing use Sterling engines.
Builds range from 600 HP naturally aspirated V8's to Twin-Turbo V8's with 1700 HP.
If that's not enough power, you can go the Blown Nitro-Methane route with engines producing between 8,000 HP - 10,000 HP

like these 400+ KM/H monsters: