Miller wants to expand the farm again, but there doesn't seem to be enough water to keep all the plants hydrated. It seems he needs more help from nearby Pokémon. This time, he is seeking the help of a water-type Pokémon. Here's how to complete the Request for Help Wanted: Field Watering in Pokémon Legends: Arceus.
You will unlock this request after completing the task of calming Lilligant in the Crimson Swamp and speaking to the Galaxy Team leader. After that, go to Miller and he'll give you the task of finding a Pokémon that knows a water-type move. He doesn't state that he needs a water-type Pokémon, so as long as the Pokémon knows a water-type move you're good.
Head to the Crimson Marshlands and hunt down one of the Psyducks. Psyducks are abundant in this area and can be found around most bodies of water during the day. Psyduck can also be found in the Heartwood area in Obsidian Fieldlands. Once you have a Pokémon with a water-type move, go back to Miller at the farms and deliver the Pokémon to him. You will be rewarded with more farmland for growing crops.