Rail-like bird — one barely entering the water (6-6)

I believe the answer is:

skinny-dipper

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

'one barely entering the water' is the definition.
I can't tell whether this definition defines the answer.

'rail-like bird' is the wordplay.
'rail-like' becomes 'skinny' (I can't explain this - if you can you should give a lot more credence to this answer).
'bird' becomes 'dipper' (dipper is a kind of bird).
'skinny'+'dipper'='SKINNY-DIPPER'

Can you help me to learn more?

(Another definition for skinny-dipper that I've seen is " Bare breaststroker?".)

I've seen this clue in The Times.

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