After getting your 8th Gym Badge, go north of the Pokemon Center in Sunyshore City and your rival will talk to you. Speak to Jasmine, Johto's Steel-type Gym Leader, after and she'll give you HM07 Waterfall.
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The HM Whirlpool is not obtainable in Pokemon Platinum, for there is no use for it during gameplay.
To get HM7 (Waterfall) in Platinum you can get it at Sunyshore City talking to Jasmine at the north of the city.
Dig is not a HM in Pokemon Platinum but a TM, it can be found in the ruin maniacs cave.
In Pokemon Platinum, Flash is a TM.
Surf is HM #3. :-)