I can only comment on St Lucia, St Thomas, Antigua and Barbados. All good to do independently by taxi. Never really saw anything apart from the beaches and shops in St Thomas but comfortable enough to walk around the shops on my own [hubby doesn't do shopping]. St Lucia - about an hours drive to the Pittons area where the 'volcano' is [also a lovely garden down there] and you can make stops on the way. My favourite on St Lucia is Pigeon Island which is a short hop from the dock on a taxi [mini buses to share or normal cabs]. It is a protected area which is scenic with a beach and various walks also cafes - small fee to get in the reserve.
In Antigua the beaches are good but beware jellyfish! Nelson's Dockyard and Shirley Heights [in the same area] are very interesting to visit by taxi [again you can share which is good value].
Barbados is a great island and a general tour by taxi is good. The contrasting coasts and great scenery make a good half day out if not more. We once went to a plantation house which was interesting historically.
Hope to have been of help please email me if you want to know anything specific, sorry I don't know anything about snokeling