Swimmer's base varied on a ship

I believe the answer is:

seabass

'swimmer's' is the definition.
(sea bass is a kind of fish)

'base varied on a ship' is the wordplay.
'varied' indicates an anagram.
'on' indicates putting letters inside (as in clothing 'on' a person).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'base' with letters rearranged gives 'seba'.
'seba' placed around 'a' is 'seaba'.
'seaba'+'ss'='SEA-BASS'

I've seen this clue in The Daily Mail.

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