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.
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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.
You cannot catch Wailord. You can, however, catch Wailmer, which evolve into Wailord starting at level 40.
Evolve a Wailmer (Level 40)
Swampert or Wailord.