They take their turns at sea (5)

I believe the answer is:

tides

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'turns at sea' is the definition.
The definition and answer can be both to do with motion as well as being verbs in their -s form.
Perhaps you can see an association between them that I don't see?

I don't understand the remainder of the clue.

Can you help me to learn more?

(Other definitions for tides that I've seen before include "Diets (anag.)" , "Lunar-driven ocean movements" , "Oceanic movements" , "Critical moments" , "Sea movements caused by moon" .)

I've seen this clue in The Globe and Mail.

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