Any professional Captain risks his livelihood, license and vessel if they ignore the simple rules of C&I on International voyages.
I commonly meet charters on vacation in a rental boat who flaunt the law either through nonchalance or pure ignorance - I can only hope they don't ever have to suffer the consequences.
With all the modern computer innovations an incident involving this transgression will result in all persons involved having a red flag on their passport and a series of hassles at any US border crossings for quite a while as I understand it.
It is a basic rule worldwide to clear C&I when entering another country.