You can get a Poliwag / Poliwhirl only after defeating E4, not before.
Similarly, Fearow, Raticate, Banette, Makuhita (in swarms), Camerupt, Skarmory, Ductrio, Cacturne, etc can be caught only after completing your Regional Dex.
After Completing your Regional Dex, you would go to the Battle Zone via Snowpoint City. You would head north and proceed till you get to Survival area. On the way to Stark Mountain, at routes 227 and 228, Fish or Surf to get a Poliwag / Poliwhirl.
As they can be encountered only after E4, they are not in the Regional Dex of Platinum. So if you are planning for Gyms and E4, you should choose any other water type. Such as Floatzel and Gastrodon.
Hope this helps :)