When you arrive, most things have closed shop. The Pokémon Center is still open; it’s to the northeast of the Dive spot. You can use the Dowsing Machine southeast of the Center to find a Protein behind the house.
For now, when done, go northwest of the Dive spot (west of the Center) and land, then use the Dowsing Machine to find the Super Potion near the tree. Get on the shore a bit to the north of there and ascend to the Poké Mart if you want to restock. (Ultra Balls are a good idea.)
Go along the path north of the Mart to find Steven meeting with Wallace, the Sootopolis Gym Leader. They’ll take you down to the entrance of the Cave of Origin, where Kyogre/Groudon is supposed to be.
In front of it, you’ll be given the Magma Suit in Omega Ruby and the Aqua Suit in Alpha Sapphire, which will aid you in your way to meeting your fate, alongside the Blue/Red Orb you have. With everyone’s words of encouragement, you are thrust to your fate. Enter the cavern.