The Ocean Mariner departed Columbia with the stated destination of the Dominican Republic. But it started sailing towards Cuba. When it realized that it was being tracked by the USCG, it changed course towards the Dominican Republic.

We don't know with certainty what it's intent was, but it's likely it was trying to sell oil to Cuba surreptitiously, so as to avoid triggering retaliatory tariffs against Colombia.

The ship was free to dock and offload in Cuba, but that would trigger tarrifs against Colombian exports the US. Which is why it turned around when it realized it was spotted. All the coast guard did was ensure that the ship docked at its stated destination.