In RSE, it's found in the underwater routes 124 and 126 (65% chance) in all versions.
In ORAS it's found on underwater routes 107, 124, 126, 128, 129 and 130 (30% chance) in both versions.
To get the Deep Sea Tooth and Deep Sea Scale, which are needed to evolve your Clamperls into Huntail and Gorebyss to complete the dex in both versions:
RSE: Find the scanner on the abandoned ship on route 108 (east of Dewford town and route 107) and bring into the Slateport shipyard to Captain Stern. He will give you the option between the scale and the tooth. You can only get one in each playthrough.
ORAS: Found in different Mirage Caves/Mountains that appear daily using the Dowsing Machine. The tooth is located in the caves and the scale is located in the mountains.
Sources: RSE Clamperl | ORAS Clamperl | Deep Sea Tooth | Deep Sea Scale