If you plan to bring a car over, you can take your chances and play the line game. Or call for a reservation on the boat (at this time get on off the island!) - HINT: these tickets are expensive from the Rockland side - so let me know if you plan to do this so I can try to get tickets for you. From this side - you have to be in line 15 minutes prior to boat departure.
If you intend to bring a vehicle to the island, you will need to call to get a line number off the island. You must call the day before you wish to leave, lines open at 530am and request which ferry you would like to leave on. You may only get a stand by. Or call and get a reservation a month in advance for that day. So to leave on July 23, you need to call on June 23 and request a reservation. If you would like me to do this, please ask!
Ferries leave 7:00, 8:45, 10:30, 1:00, 2:45, 4:30