https://www.gamespot.com/articles/new-pokemon-go-water-festival-event-going-on-now/1100-6459482/

Pokemon Go's Adventure Week event may be over, but a new in-game event is now underway for a limited time. From now until June 21, players will be able to take part in the returning Water Festival, which offers a variety of bonuses, along with another chance to catch a powerful Legendary Pokemon.

Among other things, Water Pokemon such as Magikarp and Wailmer will spawn more frequently for the duration of the event. On top of that, 2km Eggs will hatch into Totodile, Mudkip, Corphish, and other Water-types, and every Egg that's hatched will reward players with double the usual amount of Candy and Stardust.