@Tsukento:
So all of that heavily outweighs the old "Well they have Marine written in English!"
Ridiculous.
Anyone who uses context over what's actually written is looking way too deeply into things. That goes for Ubiq too.
Kanji is used for the benefit of the (younger) Japanese to add meaning to an otherwise meaningless English word. Oda is not trying to be "reverse wapanese" with OP and make everything Western, his world is a very Japanese one with a lot of Japanese ideas of Western culture from various time periods. There isn't a Western civilization that closely fits the world OP is set in, it's a chimera of all those ideas. Hence, he uses kanji, versus just using katakana for all his names and expecting people to not get it.
Oda is calling his Navy "Marine". Is is the same as calling America's Navy "The United States Navy" or the British navy "The Royal Navy".
It is pretty evident Oda's "maritime forces" are a combination of Marine and Navy. The "sailors" are completely amphibious, fighting both on ocean and land terrain. They're rifleman and can man a sail, two jobs that have never been combined in modern maritime militaries.
English is always the DEFINITIVE example of what something should be called for our purposes. This is followed by katakana romanji, then kanji last. Kanji is for the meaning/understanding, not the naming convention.