In Platinum, Diamond, and Pearl they removed Dive.
you can't
Dive is not an HM move in Pokemon Platinum. There are, however, Pokemon that can learn it by level up.
no
going to the Pokemon league on a sandbar
it isn't in Pokemon platinum its in ruby and Sapphire
Gyarados cannot learn Dive by itself from leveling and must learn it from the HM Dive which is not available in Pokemon Platinum. The HM for Dive is only available in Pokemon versions; Ruby, Sapphire, Emerald, White, Black, White2, and Black2
Dive is absent in Pokemon pearl,diamond,platinum,heartgold,and soulsilver . But will return in Black/White =-)
There is no HM for Dive in SoulSilver, but you can get it by transferring a Pokemon who knows dive into SoulSilver the Pal Park. but you can get dive as a HM in Pokemon black and white but not in soulsilver,heartgold,platinum,diamond,and pearl
in Pokemon platinum if you have a water type Pokemon go to the tutor men in pastoria city
on your way to victory road, in the water you will see a poke ball that is a dive ball
first you will need a grotle lv it up to lv.25 then talk to pr.rowan then he will give it to you (only Pokemon platinum)