U menya nikogda ne byvaet malo idej pro snorkeling :))

I guess I would start with Grand Cayman, Aruba and St John (flight to St Thomas) if I were you. Grand Cayman is really not pretty but comfortable. St John is very very pretty but not developed. Aruba is hot and kind of far away but beaches are good and snorkeling is good. Plus there are non-stop flights, although probably very expensive ones.

I guess I would not go to Turks & Caicos. Not in December.