'Marine' is also an adjective that refers to the ocean, similar to 'maritime'. I think 'marine' is much more common than 'maritime'. The latter is usually, but not necessarily, used to refer to human activities related to the ocean.

Navy: A branch of the military that usually serves on the water in ships.

 Ex: "I am going to the Navy." Or "The Navy is very important".

Marine: A marine is someone who is in the "Marines", another military branch. They are responsible for being stationed under the sea like in a submarine.

 Ex: "I am a marine because I serve in the U.S. Marines."

Maritime: refers to objects or any activities related to the sea.

 Ex: Maritime animals: animals in the sea

       "I enjoy maritime activities." Meaning I enjoy acitivies on or around the sea such as boating, swimming, fishing, etc.

I can't say thanks any more for what you done ,and thank you so much for you excellent explanation.