When you ask if it's safe, how do you mean? Will people bother you? Will you be vunerable to criminal activities? Probably not.

But is it really safe to hike or snorkel alone? What happens if you get into trouble, get a cramp, fall.

I would try to hook up with other people.

My 2 cents.

There aren't any areas of STJ that are considered dangerous. The island is quite safe, but certainly not crime-free. You should always stay aware and take normal precautions.

Maho would probably be an excellent place for a solo traveller. The people who are drawn to stay there are generally pretty nice folks. There are a lot of opportunities to interact with others in a comfortable way. Most nights there is some program or activity. There are excursions (for a fee) to different locations on the island. There's a shuttle (for a fee) that runs between Maho and Cruz Bay that could drop you at any of the north shore beaches. There are a few good snorkeling beaches in easy walking distance.

There are a wide variety of trails on STJ---ranging from strolls to demanding hikes, some well travelled, some remote. It's always safer to snorkel with others, but very unlikely you'd be preyed upon.

I have a many times traveled to the islands by myself, tho I don't do foolish things, by all means I dont swim or snorkel alone, the ocean has currents that can be tricky. I also have hiked, walked and run in certain areas alone many times and not once has anyone aproached me. I am such a tomboy that hiking alone is enjoyable, but then I could probably out run someone trying to get me. Maybe that's false security, but that's my 2 cents.
Have fun