If you hold only a foreign pilot license, or a U.S. pilot certificate based on your foreign pilot license, you will need to follow the instructions on the FAA website in order to add a U. S. single engine sea rating.
Go to the following website: FAA Website for Foreign Pilots.
Then scroll down half-way to: Verify the Authenticity of a Foreign License, Rating, or Medical Certification .
The forms you need are at the bottom of the page:
Remember, the FAA would like you to apply ninety days prior to coming to the USA. The letter you receive from the FAA is in effect for six months.