I am very certain no-one cares if you take a swim - it certainly falls under personal exercise. The idea, I am certain, is to prevent social gatherings and transmission of viruses. You can't very well do that while swimming in the ocean. I believe the regulations are in place so that the authorities will have something to point to if problems with people gathering arise.

This morning I watched a whole bunch of young people piling into a powerboat to go for a day of partying. Not everyone is taking this all that seriously.

Interestingly, scuba diving was discouraged yesterday in Martinique (and perhaps other places as well) because should you have an accident, there would likely not be hyperbaric medical facilities to care for you.