in my opinion id say iron island cos there are level 30-40 rock type Pokemon in there. it got my empoleion from a level 40 to a level 59 there and it learned drill peck and hydro pump
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
Train them to a certain lvl and they'll evolve. A few needs a link trade for evolving and some can evolve by using a stone (like fire stone, water stone etc.)
in Pokemon platinum if you have a water type Pokemon go to the tutor men in pastoria city