How do I get from Beijing to the cruise ships in Tianjin.

Do you mean the Tianjin Port in Tanggu, Tianjin?
If so, you need to take the bullet trains from Beijing South Railway Station to Tianjin Railway Station. Then you can take a taxi to reach the Shiyijing Road Station of the light train. Take the light train from this station to reach Shimin Square Station, which is very close to the Tianjin Cruise Port. After that, you need to take a cab to reach the Tianjin Port.