I think the generous claim would be that given that Puerto Rico has a manufacturing dominated economy, if foreign flagged vessels could pick up goods there and

I think the generous claim would be that given that Puerto Rico has a manufacturing dominated economy, if foreign flagged vessels could pick up goods there and take them on to the USA, they would be more likely to build routes that included them. Eg you could go through the Panama Canal, stop in PR to drop off goods from Asia and pick up goods from PR, and then continue on to Savannah/Charleston/Newark. I'm personally skeptical of that.