Marine creature ditching lake — for this? (3)

I believe the answer is:

sea

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This could be a double definition.

'marine' is the first definition.
(both can mean 'of the sea')

'creature ditching lake for this?' is the second definition.
The definition suggests a singular noun which matches the answer.

Can you help me to learn more?

(Other definitions for sea that I've seen before include "Large body of salt water" , "E.g. Yellow, Red area" , "Large expanse of water" , "& 14 Ozone" , "E.g. Baltic" .)

I've seen this clue in The Times.

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