OP is correct.
Yacht crew are treated differently than commercial crew. If the ship they are working on is not a US flagged vessel, and not considered a commercial ship, B-1 is the correct visa.
f. (U) Yacht Crewmembers May be Classifiable as B-1 or D Crewmember:
(1) (U) In General: Yacht crewmembers may provide services on board a recreational or commercial yacht. Yacht crewmembers who will provide services on board a recreational vessel traveling to the United States are classifiable B-1. Those who will provide services on board a commercial vessel and who meet the requirements under INA 101(a)(15)(D)(i) above are classifiable D.
(a) (U) A recreational vessel is primarily used for pleasure by the owner or a guest of the owner. Some private yacht owners maintain crews year-round even if the owners and/or the owner’s guests only use the yacht for periods of the year.
(b) (U) A commercial vessel is primarily used for commercial purposes, such as for transporting passengers or cargo for payment or other consideration.
https://fam.state.gov/fam/09FAM/09FAM040208.html#:~:text=Yacht crewmembers who will provide,i) above are classifiable D.