Well, if you are talking about trading then you just trade whatever Pokemon you want from a diamond or pearl game and trade for it.
And if you're talking about finding them well, you have to use surf on lakes or oceans or just fish with your fishing gear.Or you go into a rainy spot and your bound to find 1.
Caterpie
KYOGRE and PALKIA.
underground
there is none
On water.
The water Pokemon gym leader is in Pastoria City.
the three starter Pokemon are the same in platinum as diamond and pearl. They are chimchar-fire, piplup-water, and turtwig-grass.
in the water by were you can find shaymin
it doesnt exist in Pokemon
pastoria city
in Pokemon platinum if you have a water type Pokemon go to the tutor men in pastoria city
The Underground