The Yemeni forces showed video, at least with Magic Seas, that they contact the vessel, inform them that they are violating a blockade, order them to slow their vessel and divert course or they “will be responsible for the lives and safety of their crew and vessel”, and repeat this warning multiple times. These BBC and American articles never mention that part, so I was surprised to hear it, but it seems the vessels are given plenty of warning and opportunity to avoid being sunk.