There are various reasons that Relicanth is hard to catch. For example, the area where it can be encountered can move.
Chat with our AI personalities
You can catch it when you use dive.
You find relicanth underwater near sootopolis city.
When you're in the Sealed Chamber, Relicanth should be first and Wailord should be last.
erwater cavern surrounding sootopolis city
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.