How is reaching Kyogre different from finding Groudon?

Same idea, wetter route in. Marine Cave surfaces on a route drowned by Kyogre's downpour, so you'll need Surf and Dive to reach the entrance instead of just walking up. Check the Weather Institute for the flooded route. Kyogre sits at the back at level 70, matching Groudon.

Can I really catch both weather titans on one Emerald cartridge?

You can, which is the big draw of Emerald. The two caves never appear at the same time, so you catch one, set off the other's weather, and track down its cave afterward. A single playthrough ends with both Groudon and Kyogre, no trading or second copy needed.

Do I actually need Flash in Emerald's caves?

Almost never. You can feel your way through the dark stretches, and the cave HM that actually earns a party slot is Rock Smash, not Flash. Flash only matters for a couple of black rooms late in Victory Road, so don't waste an early move slot on it.

Is Rock Smash worth bringing into caves in Emerald?

Yes, more than Flash is. Smashing the cracked rocks in Granite Cave turns up a Nosepass you won't meet any other way, and the ones in Victory Road hide Geodude and Graveler. Keep one party member who knows it so you're not backtracking when a breakable rock shows up.

Emerald Marine Cave

The watery twin of Terra Cave, surfacing somewhere new each time a downpour floods the post-game routes. Track it down and Kyogre is waiting in the drowned depths.

Marine Cave is in Hoenn. In the story it comes after Terra Cave and before Artisan Cave.

MARINE CAVE MAP

Emerald

Marine Cave Walkthrough

0 trainers 0 items 0 species

Marine Cave appears only after you finish the main story, and only during strong rain on one of four routes.

1From the north entrance, go west, then south through Marine Cave past the bend, and west along the bottom of the cave to battle Kyogre.
Legendary encounter
KyogreLv 70
Post-E4; appears when strong rain weather shows on Routes 105/125/127/129

Pokemon Encounters

✦Special encounters1

#382Kyogre

Legendary Lv 70

Legendary

#382

Kyogre

Water

Level

EV yield

3 Sp. Atk

Acquisition

Legendary

Appears in Marine Cave after becoming Champion — location rotates between underwater routes

at Room 2

Sealed locationRoom 2Marine CaveView on map

One-of-a-kind. You only get a single shot per cartridge — knock it out or run, and it is gone for the run. Save before engaging.

Kyogre page →

Marine Cave Guide

Same idea, wetter route in. Marine Cave surfaces on a route drowned by Kyogre's downpour, so you'll need Surf and Dive to reach the entrance instead of just walking up. Check the Weather Institute for the flooded route. Kyogre sits at the back at level 70, matching Groudon.
You can, which is the big draw of Emerald. The two caves never appear at the same time, so you catch one, set off the other's weather, and track down its cave afterward. A single playthrough ends with both Groudon and Kyogre, no trading or second copy needed.
Almost never. You can feel your way through the dark stretches, and the cave HM that actually earns a party slot is Rock Smash, not Flash. Flash only matters for a couple of black rooms late in Victory Road, so don't waste an early move slot on it.
Yes, more than Flash is. Smashing the cracked rocks in Granite Cave turns up a Nosepass you won't meet any other way, and the ones in Victory Road hide Geodude and Graveler. Keep one party member who knows it so you're not backtracking when a breakable rock shows up.