Well the only cave close to fortree city is scorced slab cave it has no Pokemon it only has the TM"sunny day" so sorry dude/dudat.
Get the leg pokemon mewtwo
You Get Cobaloin In A Cave Near Where The Old Man Talks To You About Legendary Pokemon. You Must Have A Pokemon That Knows Surf Though.
complete the sealed chamber puzzle in route 134 ? (maybe) then the caves will open in the desert near lavaridge town, the island cave upper left of dewford town and the other cave between fortree city to lilycove town (near scorched slab)
in tin tower in ecruteack city or the cave near the whirlpool
There are several Legendary Pokemon in Sapphire, but I guess you're referring to Kyogre. He can first be found in an underwater cave near Sotopolis and after you've found him, he will travel to the cave of origins where you will get the chance to catch him.
use the super rod near the weather center near fortree city in the water.
go near fortree city and find stiven!
You can't catch Raiku if you picked Bulbasaur. Entei will be found at random
You go into a cave near hearthome city and orgboue city and use surf then rock climb. From there on good luck! PS your on your way to legendary Pokemon
you need a Pokemon that can surf and use whirlpool first. After you have all that, you surf in the ocean near Oviline City. You'll eventually find your way to Cianwood City and there, you'll find one of the legendary Pokemon near the cave in the city!
Firstly, go to the weather institute near Fortree city. Then talk to the scientist on the near the computer and he will tell that it's rainy in Route ... Or that it is extremely hot in Route ... When he says hot, it will be Groudon. Just search for a cave that you've never seen before, and go to Groudon who will be awaiting you after you are in the middle of the cave. Note: When there's a text with: The rain faded away, or something like that, it means you just have to go back to the weather institution to hear where the legendary Pokémon is now.
Go speak to the weatherman near Fortree then he will tell you a route (it is randomised). Go to the route (which should be on water). Look for a smallish dive spot that wasn't there before. Once your under water there is a cave called Marine cave and he/she should be there :) hope this helped