Do you need a reservation for the ferry/boat that goes to the 5 villages in CT>
I believe that it is first come, first aboard. We just stood on the rock and got on board!
We didn't need any reservations. It's a fairly large ferry. In Monterosso, they have a ticket booth at the entrance to the ferry dock. The ferry website is http://www.golfoparadiso.it/index.htm
Note that if the seas are rough, the ferry will not pull completely into the docks in Vernazza, Manarola or Riomaggiore. When we were there last May (late May), the ferry didn't even run because of the rough seas. It finally did on one of our days there but it didn't pull into the other CT towns.
The ferry does go to Portovenere which really should be a 6th CT town. It looks just like the others and a very nice place to have lunch.
You can buy your ticket and board at any of the towns except Corniglia. It only goes to 4 towns, not 5 It costs 12.50 Euros and is well worth it. It will cost extra to go to Portovenere, even though it is listed on the schedule.
We just picked up a schedule from our hotel and headed down to the docks and bought tickets to board. It is a great trip. You can hop on and off, at least wth the tickets we bought.