There are no internal flights but roughly every fortnight one of the two government-owned ships visits each of the outer islands, stopping for anywhere from an hour to a day depending on the tides. The southern trip to Nukulaelae and Niulakita takes three days; the northern trip visits two or three of the six islands up that way in four days. If you decide to stop off at one of the islands, you'll probably be there at least two weeks or maybe much more before a ship returns.
Distances from Funafuti are 112 km to Nukulaelae, 221 km to Niulakita, 102 km to Nukufetau, 118 km to Vaitupu, 242 km to Nui, 301 km to Niutao, 352 km to Nanumaga, and 400 km to Nanumea.
The 58-meter Nivaga II was donated by the British government in 1988 to mark Tuvalu's 10 years of independence. It has three twin first-class cabins with private bath and eight second-class cabins with shared facilities. The 120 deck passengers must bring their own mats, eating utensils, food (unless meals have been ordered beforehand), and a generous supply of bottled drinking water.
In 2002, Tuvalu got a second interisland ship, the 50-meter MV Manu Folau, as a gift of Japan. This ship has two four-bed and three two-bed first class cabins with private bath. The two four-bunk and seven two-bunk second class cabins all have shared bath. Third class is either a large air-conditioned room with a capacity for 40 persons or space outside on deck. A proper cafeteria is available for meals.
The first class fare on either vessel is three times the deck fare, plus a daily surcharge. Second class is double the deck fare, plus a surcharge. Meals are extra. Tuvaluans pay cheaper fares, so always make sure you're being quoted the right amount.
No alcohol is sold aboard, though it's okay to bring your own (no refrigerators available). Don't offer booze to the crew, however, as disciplinary action would be taken against them if they imbibed. If you must drink, do it in your cabin.
Since the schedule varies a lot, the only practical way to arrange passage is to wait until you get to Funafuti, then visit the Department of Marine and Port Services office (weekdays 0900-1200 and 1330-1500) in the Government Offices to see what's available. If you happened to connect with a trip, you could then change your return flight reservations.
In busy periods women have a slightly better chance of getting cabin space than men, but even if you've booked and paid for a cabin you're not safe until the ship has actually sailed. VIPs such as the prime minister or a member of parliament have the right to requisition your cabin on short notice, and all the kicking and screaming in the world won't help a bit. As you'll be told, these vessels are heavily subsidized and not designed for tourism. Be prepared for a lot of seasick fellow passengers.