I have just visited the Andaman and Nicobar islands, and I also had such issue when I was booking a ferry between Havelock and Neil. The ferry schedule may also be erratic, and not every time one is able to make a reservation to have a ticket beforehand. I found myself reporting at the ferry terminal early in the morning and queued to purchase a ticket. When doing my research about the topic, it happened to me that the following destination, Andaman and Nicobar islands budget packages​, can be helpful in terms of booking ferry tickets beforehand, which can save you a tremendous amount of time. This can particularly be useful during the high season when the ferries are likely to be busy and the tickets can be sold out very fast. My personal advice would be to end up with a recognized tour operator who will assist you in making your ferry ticket and other tour time arrangements. They may also be very helpful in terms of suggestions and insights about the local culture and customs. Traveling between the islands can be stressful and very hassling to do but by planning and being adaptable, you can reduce this. Also, it is advisable to monitor the weather conditions before travelling, as sometimes the sea may be rough and obtain a schedule of the ferry.