There always seem to be lots of local taxi operators at the dockside so these usually give the best deal. Take US dollars as they are widely accepted in the Caribbean and it saves changing to local currency. Your flight time will be around 10 hours.
If it is your first time I would say go on organised trips on the other hand if you meet up with people who have been before(and no doubt you will)you could hire a taxi together etc. They say Tortala is good for dolphin swimming!7 hours I think but we flew to Bridgetown.