My guess is that he is talking about either Sootopolis City (the place with the 8th gym leader) or the cave of origin, where Kyogre is.
Near sootopolis city.
In the dark spots in Route 128, use dive.
use pikachu
Underwater cavern.
The Seafloor Cavern is in route 128, south of Mossdeep City. You need to use dive to access it.
i think you get to the underwater cavern after beating the 7th gym leader
There are loads of legendary Pokemon in Sapphire, but if you mean the main one, Kyrogue, then it is the Underwater Cavern.
The Adamant Orb can be found underwater in Route 128 on the Sealed Cavern when diving in the middle.
You can only enter that cave after finding team aqua in the underground sea cavern (somewhere underwater) and defeating their boss.
You can get in after you beat Archie at the route 128 cavern.
It's a short surf from Mossdeep City, on Route 128. Wait for a large dark spot on the water, then DIVE, DIVE, DIVE! It should be somewhere along that Underwater section.
Their base is always at Lilycove, but you can find them other places (Mt. Pyre, Underwater Cavern, etc...).
you have to beat the mossdeep city gym and fight team magma in the space center then you get dive from steven teach it to a Pokemon go to route126 surf until you find white rocks then you find a dive area and the a little cavern after a little exploring go into the cavern go up and voila
Go to route 115 and use dive in the dark water then look for the cavern go inside then use dive again and you will appear in the cavern.