Recommendations - Government ferries are way better than the private ferries and are easily available, you will find many YouTube videos and websites telling the wrong information saying that "the Islanders are given preference over the tourists and you will have to stand in queues since 6.00 am and still almost impossible to get the ferry tickets, and govt ferries take 4 hrs while private ferries take only 2 hrs" - It's all wrong and misleading.
Just go to the DSS counter at any time between 8.30am to 4.pm, you will definitely get the tickets. Govt ferry ticket charges to any island are same - 510 rs per head.
While the private ferry charges are around 1300 to 1500 rs per head.
Govt ferry services are not available for Ross island and North Bay island, there are frequent boats sailing to these islands, charges per head 600-700 rs return.
Ross island is a must visit,
North Bay island can be avoided, it has nothing much except that light house printed in 20 rs note.
Cellular jail - Light show can be avoided, it is time consuming and not at all interesting.