Black captain almost straddled watercraft (3,3)
I believe the answer is:
jet ski
'straddled watercraft' is the definition.
(I've seen this in another clue)
'black captain almost' is the wordplay.
'black' becomes 'jet' (similar in meaning).
'captain' becomes 'skip' (short for skipper).
'almost' means to remove the last letter (most of the word but not all of it).
'skip' with its last letter removed is 'ski'.
'jet'+'ski'='JET-SKI'
(Other definitions for jet ski that I've seen before include "Vessel" , "small and speedy craft" , "Transport" , "Snowmobile" .)