· It might be a good idea to fly to the nearest set of islands (Fiji is a good one) and go from there. Cargo ships, mostly small, travel from the main islands to the outer islands, but if you're fixed on time limits you'll have difficulty.
You cannot book ahead, you must go to the harbour and ask around for what's going out. There's not a timetable exactly - I waited for one boat that was 24 hours late!
But it's well worth doing. If you're tough enough and have a good sleeping bag, ask for deck tickets and chuck down with the islanders. If you're there between May and September it's the yachting season. In Fiji go to the yacht club and check out if you can hitch a sail on a yacht. You'll be expected to contribute towards food and booze and give a helping hand on the boat.
Also try yachts out of Vavau, the northern island of Tonga.
Karen Steele