the cheapest way would be to get the hourly minvan that runs from Krabi town to Koh Lanta for 350thb, if you have somewhere to stay on Lanta you can ask them to arrange for the van to pick you up at the airport on the way past, or you can take the shuttle bus into the town and get the van from there. Taxi's are plentiful at the airport and there's both an information desk and a transport desk who can help you with transfers. A taxi direct to Lanta costs from 2200-2800thb. There is a ferry to Lanta from Krabi town pier, it leaves at 11.30 and costs 400thb, so if you are on one of the early flights, you can take a taxi to the pier and catch the boat. It is very easy to get to Lanta and not scary at all, people will point you in the right direction. I don't know anything about the speed boat transfers other than they are expensive and only part of the journey is by boat, with the bulk of the trip in a car/van