In Frederiksted on the west coast of St Croix USVI, the snorkeling at the old Frederiksted Pier is worth checking out. The old pier was mostly remove above water, but underwater there are several structures still standing that are encrusted with marine life including coral and sponges.
The new pier can also be an interesting snorkeling dive to spot sea horses and Caribbean lobster. Just make sure it's an off day for the cruise ship arrivals!
Here are some general notes about traveling around St Croix. First, if you plan to be snorkeling at multiple dive sites on St Croix, you will want to have a rental car so that you are not at the mercy of the St Croix Taxi Association. Those taxi costs, especially for multiple people, add up quickly. When renting a car, you will want to make your rental car reservation several months in advance to make sure there is actually one available when you arrive. Multiple reservations might not be a bad idea, but just make sure to cancel the extra reservation when you actually get your car. There several popular island-only rental car companies, too. Second, do not leave your valuables unattended anywhere due to theft on the island. The hotel safes are known not to be all that safe, either. Carry valuables with you or leave with someone you trust. Third, make sure to arrive on island with low expectations for the beach resorts, especially, the Carambola Beach Resort and the Chenay Bay Beach Resort. Check out the hotel review websites and make your own educated decision.
Overall, Frederiksted Pier is the 3rd most popular snorkel dive spot of all 10 snorkeling dives in St Croix.