It’s only a 45-minute flight from Oahu to Kauai, but they managed to sneak in a POG or water beverage service (passionfruit-orange-guava, a staple in the islands, apparently. Yummy).
I was sort of bewildered that there aren’t ferries running between the islands. Apparently, someone did try to set up a ferry service from Oahu to Kauai, but the locals were very much against it, and every time the ferry came to dock, they’d block the harbor with their boats; eventually, the company gave up.
But the bigger problem is the ocean — it can be quite rough, with big enough swells that a ferry would really struggle. So you might have to wait 3-4 days for your ride…
…that said, my friend says that if you know someone with a boat, you generally CAN sail from island to island.