The official Pokémon website has announced that from July 1st, 2022 until July 10th, 2022, Max Raid Battles in Pokémon Sword and Pokémon Shield will feature Poliwag, Marill, and Spheal appearing more often in Pokémon Dens with a chance for Shiny Marill to show up.

Defeat Water-type Pokémon to Earn Refreshing Beverages in Max Raid Battles

Make a splash and chill with Poliwag, Marill, and Spheal in the Pokémon Sword and Pokémon Shield games.

Cool off this season with some refreshing Pokémon Sword and Pokémon Shield Max Raid Battles. Until July 10, 2022, at 4:59 p.m. PDT, Poliwag, Marill, and Spheal will appear more often in Pokémon Dens. Lucky Trainers might even encounter a Shiny Marill! Defeat these Water-type Pokémon to earn refreshing items like Fresh Water, Soda Pop, and Lemonade.

For an opportunity to encounter these Pokémon in a Max Raid Battle, you'll need to connect your Nintendo Switch system to the internet through Y-Comm or by selecting Mystery Gift in the X menu, then selecting Get the Wild Area News. After that, whenever you see a pillar of light shining from a Pokémon Den, you can walk up to the Pokémon Den and interact with it to start a Max Raid Battle with a Dynamax Pokémon. This event does not affect the Pokémon encountered within Dynamax Adventures.

Source: https://www.pokemon.com/us/pokemon-news/defeat-water-type-pokemon-to-earn-refreshing-beverages-in-max-raid-battles/