We've never had any problems anywhere in the Caribbean BUT we normally only cruise in the winter months. There were some "sand fleas" in Roatan this past January but 100% Deet for Outdoors took care of that.
For Sea Lice and Sand Crabs there is a product called "SEA SAFE". Friends with a house in St. Croix tell me it works great. (Like you, I was interested and wanted to know if there would be a problem. )