You got a good response from someone else below, but here is an example of insanities Jones Act leads to. There have been companies running LNG vessels, sailing from Russia to Northeastern US, selling Russian natural gas there, and then sailing south to buy American natural gas in southern states for much lower price, and than sailing back east to sell it in Europe or further east. Northeastern states buy expensive Russian gas instead of cheap American gas, because there is no vessel that could ship gas from south to north: because of Jones Act, it would have to be American built American operated vessel, and such vessel quite simply does not exist.
The Jones Act[0] requires that goods transported by water between US ports to fly under the US flag, use ships constructed in the US, owned by US companies, and be crewed by US citizens. This results in a substantial cost increase for water traffic. It's otherwise quite cheap to transport goods by water but because of the act highways and trucks are used instead. An OEDC study[1] found that repealing the act would benefit the economy from $19-64 billion.
