In emerald, there is a slim chance you'll catch one in Route 129, but the easier way is to catch a Wailmer and evlove it.
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You Can't
You cannot catch Wailord. You can, however, catch Wailmer, which evolve into Wailord starting at level 40.
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.
theres a certain spot u go and if u have wailord and relicanth(in a specific order forget sorry!), you can catch the regis
Rather than catching a Wailmer at Lv. 40, you can find one by surfing on Route 129 or Fishing on Route 136 with the Super Rod. (though they're very rare, I'd say evolve Wailmer) You can catch a wailord at route 129 by super rod. Their level could be very high, although i met a lvl 28 wailord there by accident.