Ocean swimmers not coming back on board after short race (11)
I believe the answer is:
zooplankton
'ocean swimmers' is the definition.
(type of sea creature)
'back on board after short race' is the wordplay.
'back on board' becomes 'plankton' (I can't explain this - if you can you should believe this answer much more).
'after' is a charade indicator (letters next to each other).
'short' means to remove the last letter.
'race' becomes 'zoom' (both can mean to move quickly).
'zoom' with its last letter removed is 'zoo'.
'plankton' after 'zoo' is 'ZOOPLANKTON'.
'not coming' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for zooplankton that I've seen before include "Drifters" , "Life drifting" , "seafood" .)