They have been boarding ships that fly false flags. That is, they claim to be flying under the flag of some country. But when the US contacts that country to confirm that the ship is really registered there, the government of that country replies that the ship is not, in fact, registered. This is legal to do regardless of the embargo against Cuba.
There are plenty of ships that move good and resources to Cuba that don't get boarded.
There are plenty of ships that move good and resources to Cuba that don't get boarded.