https://www.gamespot.com/articles/new-pokemon-go-gen-3-legendary-now-available-for-a/1100-6456089/

The Legendary Pokemon Kyogre is now available in Pokemon Go. Players can now encounter the Sea Basin Pokemon at Gyms in Raid Battles, where it can be found for the next several weeks.

As with other Legendaries, you'll need to team up with other players to battle Kyogre; only after defeating the Pokemon will you have a chance to capture it. Kyogre is a powerful Water-type, so players will want to bring Electric- and Grass-type Pokemon such as Jolteon, Zapdos, and Exeggutor to gain the advantage over it.