After you beat Drayden you go to his house and he will tell you about Reshiram and Zekrom. Then, Team Plasma freezes Opelucid City and you have to defeat a bunch of grunts. Next, you have to go to the Opelucid Gym and defeat Zinzolin who stole the DNA splicers. Last, you must beat a member of the Shadow Triad next to the east gate. The Marine Tube should then be open to go through.e
Also, when you get to Humilau City, Marlon, the Gym Leader, will not be there. You will have to go to the northern part of the city, when you're just about to enter the first house, he will come up out of the water and return to the Gym.
Hope I helped! :)