One can travel from Port Blair to Havelock Island only through sea. Government ferries are the cheapest option, but they take longer. Makruzz a catamaran service that runs regularly and Green Ocean ferry are the other two options.
Sailing of ferries and Makruzz is subject to weather conditions. Ferry schedules are generally published up to a month in advance. Ferry tickets can be booked only locally. They are not available online. Makruzz and Green Ocean can be booked online though.
A government ferry generally takes up to 4 hours, while Makruzz takes half the time. Makruzz is airconditioned and is quite comfortable. While traveling you should be aware of and take precautions for sea / motion sickness. Ferries are mostly used by residents to travel between the islands and they are also a way to transport goods and you will find motorcycles, taxis and cars loaded on the ferries.
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** My experiences based on travel before the COVID-19 pandemic.