Line showing equal water depths is over tub (7)

I believe the answer is:

isobath

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'line showing equal water depths' is the definition.
I can't judge whether this definition defines the answer.

'is over tub' is the wordplay.
'over' becomes 'o' (cricket abbreviation).
'tub' becomes 'bath' (synonyms).
'is'+'o'+'bath'='ISOBATH'

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I've seen this clue in The Times.

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