The government ferry system on the Andaman Islands is utterly ridiculous, but it is also by far the cheapest way to travel to Havelock Island from Port Blair A one way ticket only costs 400 rupees, and a return ticket 800 rupees.
There is a slight catch, and that is you have to buy a return ticket. This seems reasonable, as it’s likely you’re only visiting and will want to return to the airport. However, you can only buy the government ferry tickets 3 days in advance. That means if you’re staying a week or two, you cannot buy a ticket for the date you actually want to return.