The Jones Act prohibits any non-US ship from participating in US trade lanes (including Puerto Rico).

What is the Jones Act?

The Jones Act is section 7 of the Merchant Marine Act of 1920 that prohibits any non-US built or non-US flagged vessel from participating in trade between points of the United States. Named after its sponsor Senator Wesley Jones, the Jones Act also extends employer liability of US vessel employee injuries. 

Under the Jones Act, all vessels whose trading voyage begins anywhere in the US and delivers anywhere in the US (including Puerto Rico) must be built in the US, staffed by US crews, and owned by US citizens or corporations.

The Jones Act is administered by MARAD (United States Maritime Administration), an agency within the US Department of Transportation.