Where do you get HM surf on Pokemon diamond?

Answer

What you have got to do is clear the psyduck blocking the path outside Solaceon Town and then make your way through the path towards Celestic Town then battle the galactic grunt blocking the cave enterence. Once you have go in and look at the markings at the back and a lady who you have to give the old charm to will give you surf.

To use it outside of battle you must defeat the gym leader Fantina.