Comment by gnopgnip

The Jones act restricts trips between US ports to US ships. So transport between San Juan and Jacksonville needs to be a US ship. It doesn't technically restrict ships between DR or another other foreign port and PR. But the Jones act prevents a ship going from DR to Jacksonville from stopping in San Juan along the way. San Juan isn't a large enough market, so it doesn't make financial sense to make a trip only there, or to stop in San Juan then another foreign port, then Jacksonville.