Different temperature for Henry's swimmer (5)

I believe the answer is:

otter

I'm a little stuck... Click here to teach me more about this clue!

'swimmer' is the definition.
(I have seen 'Good swimmer' mean 'otter' so perhaps 'swimmer' could also mean 'otter')

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for otter that I've seen before include "Fish-eater of the weasel family" , "Aquatic river mammal" , "Water creature" , "river in Honiton" , "one that eats fish" .)

I've seen this clue in The Independent.

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