Sorrento to Capri

Everyone's particular type of motion sickness is different but I'd vote for fast ferry (jet boat). I figure the shorter the length of the trip, the less time to possibly be uncomfortable.

Note that different towns (Positano and Sorrento, for example) have different sized marinas, and the exact choice of ferry in Positano is a much smaller boat than the companies can use in Sorrento. Ferry schedules are seasonal, and some towns on the A.C. (Sorrento is not, technically, on the A.C.) don't have ferry service before sometime in April. Be sure to find ferry schedules for your month and year of visit (I don't mean that they change monthly, only that you need to read the date on the schedule very carefuly). Some towns (like Sorrento) have more than one company providing ferries, so you may have multiple schedules to download.

We bought walk-up tickets in Sorrento. I noted that the different companies are competing, and will not venture that you could leave sooner if you went to the competition across the way. They also put pressure on you to buy a round-trip for a specific boat. That sounds annoying, but the last few boats of the day are normally sold-out, so you are better off with a known space on a certain departure. (It may be less busy in April. And that might lead to smaller boats being used.) Also, even with the same company, our return from Capri was on a very much larger boat (with no outdoor seating, and hundreds of airline style seats inside an air-conditioned cabin) that left from a different slip - one that was very far from the slip where we had arrived in Capri. So check the signs at the port before you hike over to the slip that you used before.

Sorry if this makes it sound harder than it is. It isn't very hard, but I wanted to offer as much data as I have.

Edit: Typo
I've read a lot of inquiries about the "hydrofoil", but I don't know if that really exists. Maybe it's available from Naples to Capri?

We have used the hydrofoil and it is far smooth than any boat riding on the surface of the water. And it faster. If concerned about motion sickness, then take the hydrofoil but all boats have some risk of problems with weather.

Frank is right: I meant hydrofoil versus fast ferry.

Either way, I have found Bonine to be very useful in avoiding motion sickness; much better than the traditional dramamine, at least for me. Buon Viaggio!