You go to the man north of mossdeep.
Use Dive.
in the under water use dive
go to route 127. use dive. it will be there.
use dive on a black part of the water
In Pokémon Omega Ruby, you can use Dive at Route 124 in the area just south of the submerged entrance to the Seafloor Cavern. Look for a spot in the water that has a darker shade, indicating that it's a Dive spot. Once you use Dive there, you can explore the underwater area, which contains various Pokémon and items.
Use Dive with a Pokemon and Go through the water. You will see a hole. Go inside and Use dive to go to a new place.
use dive in the dark water then go into a cave. ue dive again and you should be in sootapolis city
no sadly it can only be used in ruby, sapphire and emerald
Since dive is not a HM in firered you will have to trade a Pokemon to a friend that has ruby, sapphire or emerald then using that game use the HM Dive to teach the move Dive to that Pokemon trade it back so firered can have that Pokemon again it will still have Dive however you cannot use it to dive underwater.
Use the move dive to go underwater to find shards in the seaweed.
you use dive in emerald and get waillorrd in ruby and i DONT NOW HOW TO IN SAPHIRE
Use dive on route 126 and go to the end of the under water cave therevwill be a cave use dive to go up inside of the cave and you're there