The Collaborative International Dictionary of English v.0.48:

motorboat \motorboat\ n.
   a boat propelled by an internal-combustion engine driving
   propellers immersed in the water.

   Syn: powerboat.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

motorboat
    n 1: a boat propelled by an internal-combustion engine [syn:
         motorboat, powerboat]
    v 1: ride in a motorboat

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "motorboat":
   boat, cabin cruiser, canoe, carry sail, circumnavigate, coast,
   cross, cruise, cruiser, go by ship, go on shipboard, go to sea,
   launch, make a passage, motor launch, navigate, ply, power cruiser,
   powerboat, row, run, sail, sail round, sail the sea, scull,
   seafare, sedan cruiser, speedboat, steam, steam launch, steamboat,
   take a voyage, traverse, voyage, yacht