Is the ferry easy to do?
I read somewhere that it is often too full or gets cancelled, so you have to arrive 4 hours in advance for the boat. Is this true?
What you have heard is true. The ferry is often late/delayed and if there are too many passengers, Vieques residents are boarded first often not leaving room for visitors. There are many alternative ways to get to the island that are more convenient and much less time consuming. Below is a chart that shows timelines of the alternatives and a link that explains the options in more detail: