In this sentence, The submarine, an important part of any navy's fleet, has long been used for underwater warfare. Is submarine a adjective

No it is not. "Important" would be the adjective also the word "long" I think. (a word or phrase naming an attribute, added to or grammatically related to a noun to modify or describe it) is an adjective. The submarine would be a noun. So in the sentence the green apple is on the tree, green is the adjective.