Back in Undella Town, head west of the Pokemon Center towards the previously inaccessible Marine Tube. As you head that way, Hugh will call you up on the Xtransceiver, telling you to take the Marine Tube to Humilau City.
Head inside and then follow the path to reach the Marine Tube!
This place is definitely pretty cool! There's no items or anything for you to worry about picking up, so just enjoy the walk underneath the water until you arrive on the other side!
In the gate on the other side, though, be sure to talk to the Parasol Lady to receive TM42 (Facade), which does double damage if you're affected by a status condition. Alright, now head outside to arrive in Humilau City!