Funny how everything can be turned into America.
Nevermind the many other empires, ones that actually roamed the seas such as the British, that existed thousands of years before the US even did.
Discussion
In One Piece, it's very obviously the U S. for a couple of reasons:
1. The Navy men wear American-ish style uniforms
2. The Japanese have hosted U.S. military bases, mostly Navy, since WWII.
In addition to those two reasons, the United States Navy is the main mode of power projection for global empire today. Yes, that role used to belong to the British Navy, no disagreement there. Then U S. Took over that role from the British after WWII at about the same time that it established it's military presence in Japan.