In St. Martaan I would do Pinel Island on your own.
In Barbados, I highly recommend Cool Runnings which is a catamaran cruise we took outside the ships excursions. Tiami ships were sold out and they recommended Cool Runnings. Captain Robert and his Mates are fun and very informative. Bajan Lunch and drinks fabulous. You must swim with the sea turtles and see the ship wreck.
Cost about $60 for adults --- $32. for kids 12 and under.
Robert and Anika Povey (sp) are owners try Coolrunnings.com . if that doesn't work email me and I will look for address. E