After you defeat Rayquaza and beat the "story mode", talk to Whiscash. He'll talk to you about disturbances in the ocean, and asks you to figure out the situation. After which, he'll give you the HM Dive, and Stormy Seas will be unlocked.
After beating the game it becomes available for purchase at Wigglytuff's shop
u can unlock it by drinking spinda's smoothies sometimes, but it has a very low chance u can.
electric plateau is also called amp plains. if you dont have either, you unlock after the expedition.
You have to beat Skarmory first, then go south of the Pokemon Square and either talk to the Makuhita or walk in the doorway.
beat the story and go to bed with TMSurf in your toolbox and when you waike up, the mission will start. P.S unlock the southern island too
Boner
Beat Stormy Sea
You Can't
wonder mail
i believe you have to befriend with lugia
to evolve you have to get palkia and unlock marine resort
Buy friend areas. The last dungeon is the deoxys one
talk to medicham outside our leader's house. then to lombre at the square. After that, talk to whiscash at the "whiscash pond" (above center of town square)
Not in Pokemon blue mystery dungeon but there are them in Pokemon exploreres of darknes and exploreres of time.
solve missions and you will get further and unlock new dungeons i know i completed it
wonder mail or on the job bulletin board
you don,t and you can"t