I am trying to think of a clear way to explain, so let me try it this way. Trying to make a reservation on the hydrofoils (most are actually jet boats) would be like trying to reserve a ticket on a city bus. It just is not done. You simply walk over to the ferry pier complex (right next to the cruise line pier), check the daily schedules posted clearly over the ticket windows of the various companies and buy a ticket for the next boat. My only caveat is for the return journey. I suggest that when you arrive on Capri (you will be at the port of Marina Grande) you stop at the ticket office at the end of the pier and check the various return schedules. At that time, buy your return ticket so that you do not have to waste time standing in line in the afternoon. As to the Grotto, as you walk off the pier in Marina Grande you will see a pier on your left with all kinds of signs for the Blue Grotto. You simply buy a ticket for the next boat (these are motor boats that hold 20 - 30 people) and the boat will take you around the island to the Blue Grotto. At this point you will be transferred into a small row boat, pay another fee for the entrance, and than be rowed into the Grotto. Its an outrageous tourist trap, but its something most of us do one time (nobody ever goes back a 2nd time).