We did the hydrofoil/jet boat once while in the port of Naples - think it was in 1999. Wow does that thing move! And, if memory serves: it was a wild trip. Lots of movement. It is a comfortable craft - but I don't know if it was that the bay was choppy or perhaps rain was moving in (it did rain for a few minutes while were waiting on Capri to return to the port in Naples). But I felt the movement/motion - and it takes a lot of movement for ME to feel it!
Having said that: I'd do it again in a heartbeat. Unfortunately, for our May cruise we've hooked up with a private tour of Pompeii and Amalfi coast - both areas we've seen before - but excited about doing them again. Wish there was time in port to do Capri again: It is absolutely beautiful!