I will give my vote to St Croix and in being fully transparent, we now own a place on the island.
Very good dive variety is available, from the wall that runs along the North side to the Pier on the West end (it is a great, easy shore dive but) if you take a boat out to the end (too far to swim), you can find lots of sea horses, bat fish etc. that are not usually seen. In February, I have seen the whole ocean floor covered in cushion star fish.
Crime - there was an incident 20 years ago when a local gang attacked and killed several cruise ship tourists. Other than that, most crime is resident on resident in the two “projects” - one just west of Christiansted and the other east of Frederiksted. Stay to the main areas and you are fine. Just so you know, there is little night life on the island, pretty much roll up the streets by 10 pm. .
One thing to consider, this is a “foodie” island. Small unique restaurants with excellent, eclectic menus. But not inexpensive.
Actually, I ought to keep my mouth shut, don’t want this quiet slice of paradise overrun with tourists.
It's great to get specific information from someone that really knows, thanks!