We have spoken to numerous people over the years who have played fast and loose with the rules and just 'nipped' over to Leinster Bay / Waterlemon Cay from the BVI for either a lunctime snorkel or overnight stay...without involving customs in their plans.

...I wonder if this still occurs, or whether the USVI have cracked down on it in some way?

I hasten to add, we personally have not done this.