Still water's rising in the boat (5)

I believe the answer is:

sloop

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'boat' is the definition.
(sloop is a kind of sailing vessel)

I don't understand the rest of the clue.

(Other definitions for sloop that I've seen before include "one going to sea" , "'----- John B', 1966 Beach Boys' hit" , "Type of sailing boat" , "Single-masted sailing boat" , "One-masted sailing ship" .)

I've seen this clue in the Sydney Morning Herald.

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