To find HM 05 Defog, go to the Great Marsh and pick up the pokeballs on the floor until you get it. You'll need the Relic Badge and the Fen Badge to use it. (AKA, Gym badge 4 and 5)
go 2 route 18 and surf. you'll find a ledge. hm05 (waterfall) will be on the ledge.
you do not find dialga in Pokemon pearl
hm05 is flash
You must have HM05 Flash to go through Rock Tunnel. Rock Tunnel is to the east of Curlean City. You can get HM05 by going through diglett's cave(and end up near pewter city) and going south to a buiding to which you will find Pro. Oak's aid. If you own 10 pokemon, he will award you with HM05.
You can find Aron only in Diamond.
Route 18
go 2 route 18 and surf. you'll find a ledge. hm05 (waterfall) will be on the ledge.
HM05 Waterfall is on Route 18.
HM05 Defog can be found in the Solaceon Ruins.
you do not find dialga in Pokemon pearl
hm05 is flash
you need to catch ten Pokemon
Trade with Pokemon Pearl or Pokemon Platinum
the best Pokemon you can catch in Pokemon pearl is heatran
You don't find a Krookodile in Pokemon Pearl
In platinum you don't get a HM in the Great Marsh. The HM that is there in diamond in pearl, HM05 Defog, has been moved to the Solaceon Ruins
talk to the person in the entry of great marsh. she will give you hm05 defog