Can anyone recommend a taxi service on St Maarten for a personal tour ?

They will be lined up on the pier-you won't have much choice. They make sure you take the next in line.

Consider renting a car. We have done that a couple of times. The island is small and you can tour and stop at leisure. There is a car rental agency at the port. You can also go on line and reserve one with no obligation. It will really give you your freedom to go where you want. Be sure to visit Marigot, the capital of the French side. Also Orient Bay is wonderful for a beach stop. There are also small beaches and coves off the road you may find.