You have to have Dialga. Dialga is at the top of Mt. Cornonet, you must enter in from Hearthome Cuty, or Dialga will not be there. From Hearthome, you have to climb all the way to the top, and Dialga will be at the Spear Pillars with Team Galactic.
In sunyshore city
It is sunny in Sunyshore City.
you have to get dialga after you could go to sunnyshore city.
Go up to the Lighthouse in Sunyshore City. You should see him there. Talk to him and then return to the Gym.
In Diamond, you can obtain a Water Stone in the underground in Sunyshore City.
It is in Sunyshore City.(West of the hotel place)
Sunyshore City
Jasmine on the beach in Sunyshore City.
spear pillar on diamond, pearl, and platinum.
Sunyshore city, where the last gym badge will be.
after you beat Cyrusin the distortion world
To unlock Sunyshore City in Pokémon Pearl and Diamond you have to beat Team Galactic. They can be found at Mt. Coronet.