where?

Sootopolis - Last Gym. To enter that city you need to go through an underground cavern right. This is where you will catch Relicanth. It really sucks because I had to fight/run from 30 pokemon before he even showed but he showed eventually.

So just fly to sootopolis > Swim to dark spot at the bottom of the town > Dive > Come out of the cave > run around in underwater grass/seaweed. But like I said the amount of wild encounters before getting him sucks

All you need to do is go to the area on the eastern side of the map where there's an island surrounded by a dark ring. The island should contain the last gym there is. Just dive down there and go through the seaweed. Relicanth should pop up eventually if you search hard enough.