Like the original Pokemon diamond or all the way back to Pokemon blue,red,green,etc you have to earn the privilage from a gym leader.
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On water.
Get the HM Surf then teach it to a pokemon.
You cant walk on water but you can use surf. You get surf in celestic town.
You get Surf from the elder of Celestic Town.
The Hearthome gym leader Fantina lets you use surf in Pokemon Diamond, Pearl and Platinum.