Most islands can only be reached by water. While this sounds romantic, ferry ticket offices can be utter chaos: expect hot waits (queues often start hours in advance), slow service, queue-jumping and a rugby scrum to the ticket window. Have your passport (for photo ID) and, if required, your ticket form handy. To hold your spot and advance in line, you need to be extremely firm or be a woman; ladies’ queues are a godsend, but they really only apply in Port Blair. You may be able to buy tickets the day you travel by arriving at the appropriate jetty an hour beforehand, but this is risky; government ferry tickets are released one to three days ahead, so buy them as soon as possible. Hotels and travel agents can usually book ferry tickets for you.
There are regular boat services to Havelock (Swaraj) and Neil (Shaheed) Islands, as well as Rangat, Long Island, Mayabunder, Diglipur and Little Andaman. Ferry schedules are not currently available online; ask locally or check with Port Blair's Phoenix Bay Jetty ferry information office. All schedules are liable to change.
More comfortable (and more expensive) private ferry companies also run to Havelock and Neil Islands from Port Blair; tickets are usually available a month in advance online and in person, making these the easiest option for visitors. Makruzz and Green Ocean are the main operators.
Porters can be hired at jetties (around ₹50 for an average-size bag), but if your luggage isn't too heavy, it's not a long walk to/from the jetties.