From Phuket you can access Koh Lanta by public ferry.
The ferry runs twice a day at 8:00 AM. and 13:00 PM.
It takes totally 4 hours to reach Koh Lanta and the ticket price
is around 850 THB. per person. The pier is just a few kilometers
from downtown Phuket.
Notice this ferry only runs during high season
from the beginning of November until May.