How do you get wailord in platinum?
You can capture a wailord in route 230. If you can not find it you better capture a wailmer and evolve it.
You need relicanth and wailord to catch the regis in sapphire/ruby/emerald. You cannot catch the three regis in platinum unless you have the event Regigigas (this event has now ended). If you have the special regigigas in your party, three new caves will open up in Sinnoh and you will be able to catch the three regis.
You can catch level 40-55 Wailords with the super rod at Route 223 and Route 230, but the chance of a landing Wailord on your super rod is very rare, at 5%. Level 30-40 Wailmers are much more common, at 30%. To save time, catch a Wailmer with a super rod and train it to level 40 and it will evolve into a Wailord.