Find Your Cruise
No need to do any shore excursions - all it needs is a bit of research. If you use a browser and type in the words 'message and cruise and antigua' - of the results returned you need approx 3rd link down that begins with the word antigua and contains the word 'boards'. Repeat for each island!
Personally after hours of research on the above site - I have booked to hire a car in St Maarten, to go with Antours in Dominica ($45pp for private tour for 6 people), and to use Gordon tours in Antigua for $40. Hope this helps.
If you just want to go to a local beach, then there are always loads of local taxis available which will be far cheaper than the ships excursions. All seem to take US dollars to save changing to local currency.