Confirmed
Personal watercraft
Jet ski style watercraft appear in Trailer 2 and again in the extended look.

Notes
Two seat jet skis show up in Trailer 2, including one on a trailer behind a pickup, and riding one is among the activities in the extended look.
Rockstar has not named the model.
Sources
Grand Theft Auto VI Trailer 2 (opens in a new tab)
Shows jet skis on the water and on a trailer.
Grand Theft Auto VI extended look: things we learned (opens in a new tab)
Lists jet skis among the extended look activities.
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