The Collaborative International Dictionary of English v.0.48:

Bumboat \Bum"boat`\, n. [From bum the buttocks, on account of
   its clumsy form; or fr. D. bun a box for holding fish in a
   boat.] (Naut.)
   A clumsy boat, used for conveying provisions, fruit, etc.,
   for sale, to vessels lying in port or off shore.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bumboat
    n 1: a small boat that ferries supplies and commodities for sale
         to a larger ship at anchor