>>2811409 (OP)I’ve only been to St. Croix, briefly, when a Puerto Rican friend with a small plane flew me over, I so I can’t comment on USVI beaches with any expertise. But the two or three Crucian beaches I visited were beautiful.
That said, Puerto Rico, which I know somewhat better, almost certainly offers a larger number of beaches with a greater diversity of terrain. My single-favorite Caribbean beach destinations are the Puerto Rican off-islands of Culebra and especially Vieques, which has more rocky than white-sandy beaches (although a few of their coastal stretches are indeed white sand), but has the benefit of a lot of coastlines that are all but deserted. Also some good snorkeling. It’s a sleepy, slightly scruffy rural nowheresville with minimal tourist/resort facilities, but VQS is IMO the single best Caribbean island. Culebra is objectively prettier by most standards (most mainland Puerto Ricans prefer it) and a little less rustic, but it’s more noticeably dominated by tourism.
Either PR or USVI will offer abundant turquoise water warm enough to enjoy swimming in.