There is some snorkelling in Aruba, at Malmok Beach, it was quite good, maybe not the best but there was something to see.

St Lucia does have some snorkelling, it is not correct that it is virtually non-existent. There is a marine reserve by Anse Chastanet and there is snorkelling at Anse Cochon which was also quite good. The sea is clear in this area (unless it rains, which it does in St Lucia.)

Guadeloupe also has good snorkelling but seems to be out of a lot of people's comfort zone. It is for the adventurous and those not seeking luxury accommodation, it can be inexpensive (unless the Euro exchange rate is unfavourable to the $). There is plenty to do eg hike to the volcano, visit the national park and do activities, cycling etc and many interesting towns to visit, good food, but accommodation choices are fairly basic.

Grand Cayman has the best snorkelling of the islands I have visited.