You catch it in the seaweed next to sootopolis city
no you do not have to catch a relicanth to get a rayquaza
its not the best but it can help you find the regi's on emerald and ruby & saphire
You can catch it when you use dive.
You find relicanth underwater near sootopolis city.
Ruby or Saphire
Relicanth can be found in the grass underwater..u need dive for this.
TRADE IT FROM RUBY OR SAPHIRE.
Saphire Version
You can't catch Relicanth. You have to trade from Ruby, Emerald, Sapphire or any other game that has him captured.
To catch relicanth use Hypnosis by Gengar and get lots of Dive Balls
kyogre,groudon,raquaza and jirachi
No only in ruby and sapphire and emerald.