How can you cross water in Pokemon diamond?

to cross water you have to go to celestic town (after defeating pastoria gym there will be a team galactic man follow him defeat him Cynthia will be there talk to her she will give you secretpotion use it on psyduck and you can go there) there will be an old lady in the ruins she will give you a HMSurf. After that defeat hearthhome gym and BINGO!

you can use surf to cross water