The is no sealed cave, If youmean the sealed chamber You need a wailord, relicanth, HM Surf HM Dive and TM Dig. After you beat the champion, use dive on the dive place on route 134. Once you get in the chamber use dig on the center top.
What cave?
To get into the Desert Ruins in Pokemon Ruby, Sapphire and Emerald you need to complete a puzzle. This puzzle is found in the Sealed Chamber hidden on Route 134.
use their Pokemon
The Cave of Origin is in Sootopolis
You can only get kyogre in sapphire you get it at the cave of origin.
behind the first gym
No,you can only catch it in a cave in route 111.
you can use it in any cave to get out of the cave
z
turn back cave
In Shoal Cave
Cave of Origin.