I have a question (possible spoilers)

[Ragnarok]

Ok so I just left Cosmo Canyon on the second disk after leaving the huge materia in the wierd space room. Anyways, I can't figure out what to do now. I looked on a guide and it says to get my submarine but I failed the submarine mission and went on, can I still have access to it? Also, how was a person supposed to know they needed a submarine and where to go if they don't look on a guide, was there something in the game that I missed?

[Necronopticous]

If you failed the submarine mission it will be impossible to get that particular Huge Materia, but you can still get a submarine and progress with the game by going back to Junon and going back to the Underwater Reactor through a door that was previously inaccessible due to a dog being in the way. Your sub will be red which will forever remind you of your defeat in the sub minigame, but it can do everything the gray one that you'd have gotten from beating the minigame can do.