Port Blair is an archipelago so you could understand there are a lot of ferrys that connect to the other islands. Especially Ross island is a very popular one due ot its historical significance as it was the base for the British Administrator and the Japanese coverted the site into POW camp. You can catch the ferry from Rajiv Gandhi jetty (see map). The journey only takes 15 minutes. It is wise to opt for a guide.
Ross island, Andaman