Marill's best moveset in Pokémon GO is Tackle and Aqua Tail, with 3.90 damage per second, and it can deal 54.6 damage before Marill faints. This is true when battling in neutral weather.
All Marill movesets ranked
Here is a complete list of Marill movesets in Pokémon GO, ranked by their DPS and TDO. You can change the weather and the opposing Pokémon.