STORY: "It's meant to send a clear message to the people of Poland that we stand with you, that you are safe, and that you are secure," U.S. ambassador to Poland Mark Brzezinski said at a news briefing in the port.
"Putin hates us working together, and we have never been ... working together like we are now," Brzezinski added.
Poland has played a key role in providing military and humanitarian support for Ukraine since Russia launched its invasion on February 24.
U.N. data showed that 5.03 million had fled Ukraine as of April. More than half the refugees have entered the European Union through Poland where many had family and other connections living in the region's largest pre-war Ukrainian community.
The ship is touring the Baltics and docked in Lithuania last April.