Heal your Pokemon and head back to where you battled the Grunts in Virbank City. If you haven't already done so, don't forget to head back to the Virbank Complex and cut that tree down with your new HM. You'll find a Super Potion if you do.
Now that you're back at the harbour you'll see Roxie and the captain of the boat talking. Once they're done, follow the captain inside the building. Inside, Hugh's already buying a ticket. Answer his question with the top ('yes') option and you'll board the boat! After a nice, cute little cut-scene you'll arrive in Castelia City!