The best thing to do would be to hire the services from a tour planner. This will make your life easier. The tour planner would deal with the procurement of tickets and passes and you can enjoy your time without the hassles of standing in queues and worrying about last minute rush for passes.
For private ferry service you can contact any of three:
Green Ocean You can book online, but need to book well in advance. Options other then ferry is private or sharing speedboats. But this could be very expensive... For any query regarding Andamans you can check my Andamans trip on tripoto... Feel free to ask anything... Happy travelling