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Wiktionary
buoy tender

n. 1 (context nautical English) Someone charged in looking after buoys. 2 A ship used to maintain navigational buoys.

Wikipedia
Buoy tender

A buoy tender is a type of vessel used to maintain and replace navigational buoys. The name is also used for someone who works on such a vessel and maintains buoys.

The United States Coast Guard uses buoy tenders to accomplish one of its primary missions of maintaining all U. S. Aids to Navigation (ATON).

The Canadian Coast Guard uses multi-use vessels (icebreaker and navigation aid) with the buoy tender platform.