There are good snorkeling opportunities on all of the islands you'll be visiting. You don't necessarily have to be a good swimmer to snorkel, but if you're not, I'd recommend using the vests that most cruise ships and excursion boats will have on hand. They are lightweight and you only need to inflate them slightly to provide you with enough buoyancy to make frequent rest stops while you snorkel.
Your ship may have introductory snorkel lessons in the pool on days at sea, so check that out if you want to get comfortable with the equipment before you hit open water.
A great place to snorkel for first-timers is Trunk Bay on St. John, which is easily accessible from St. Thomas. It's easy snorkeling from shore (as opposed to jumping in the water from a boat) and has an underwater marked trail identifying the corals.
I don't know much about Aruba, but Curacao has an area that is designated an underwater marine park with well-preserved coral and lots of fish. On Tortola, you may have the option of signing up for a snorkeling excursion to The Baths on Virgin Gorda. This is also a worthwhile trip. Though the day I was there, the water was pretty rough which could present a challenge to a novice. But there are also pretty beaches to explore here if you tire of snorkeling.
Definitely try snorkeling ... it's easy and lots of fun!