If you have self-planned your holiday and if your ferry gets cancelled then you will have to find a ticket to reach other islands by yourself. The chances are high that you might not get an alternative ticket as other people will also be trying to get the tickets on the ferry. With very little internet connectivity no online portal will open after you reach Andaman Island. It is recommended to go on a planned tour with a company such as Experience Andamans who will assist you during such times of difficulty.