i think its in Celestic Town Ruins. Cynthia's Grandmother will give you it.
you go up to the water and hit "a" and it will ask you if you want to use surf well i think i havent played it in a while
in wally's house talk to his dad
surf
Get the HM Surf then teach it to a pokemon.
You get Surf from the elder of Celestic Town.
You teach it surf by using the Hidden Machine.
Surf is HM #3. :-)
there are 7 hidden machine found in Pokemon diamond:- rock smash cut strength fly surf waterfall rock climb defog (not in oder)
On water.
you surf
Yes, it can.
A gymleader i think...