There are exceptions for the very few things the US authorizes to export to Cuba, but in general:

779. What are the “180-day rule” and the “goods/passengers-on-board rule”?

The 180-day rule is a statutory restriction prohibiting any vessel that enters a port or place in Cuba to engage in the trade of goods or the purchase or provision of services there from entering any U.S. port for the purpose of loading or unloading freight for 180 days after leaving Cuba, unless authorized by OFAC. This restriction is applied even if a vessel has stopped in Cuba solely to purchase services unrelated to the trade of goods, such as planned ship maintenance.

https://ofac.treasury.gov/faqs/topic/1541