Surf Right of Lilycove
go to shoal cave above mosdeep city
on Mosdeep island ,steven gives you after difeating the gym leader
Hm 08=Dive. You can get it in Mosdeep City in Steven House.
on the route under mosdeep
In Mosdeep City, inside the Space Centre, there is a man who will give you a Sun Stone. They can also be held by wild Solrocks.
Go inside the Mosdeep Space Centre and talk to the sailor guy and he'll give you a sun stone. Use the sun stone on a Gloom.
If I'm not wrong,surf north of mosdeep city
Mosdeep in emerald, Sapphire and ruby
u get beldum after you beat the champion u go to his house in Mosdeep city near the seventh pokemon gym it is on his table just take it.
by surfing!
in mosdeep
there is no mosdeep in diamond that's only in ruby, sapphire, or emerald