There's already a project testing these. However the channels between the main islands in Hawaii can be pretty rough. The water gets pretty deep (900m - 3000+m) quickly and due to geography the winds and waves get funneled there and can easily hit 5-10m high. The short distances make it appealing despite the potentially rough sea state and waves.

Except for those pesky waves that Hawaii is known for.

I see this as more applicable in areas coastal metro areas with sheltered waterways and existing ferry systems.

Yup. There's a reason that every photo of these (admittedly cool) vehicles is on a clear day, over calm water.

Jones act!