Its common knowledge that plants need water to grow and that remains true in Monster Harvest. To water your crops, use the watering can. Once your watering can is empty, head to nearby water like the river to refill it.
Watering your crops can be a tedious process. You can only water one spot at a time and the base can does not hold much water. The base watering can only hold enough to water nine crops but this can be increased by upgrading it.
With such a low amount of crops able to be watered, you will need to manage your stamina accordingly. Once you upgrade the watering can this doesn't become as much of an issue. You can also unlock the ability to build pipes and create an irrigation system. This will relieve the need to use stamina to water crops.
Additionally, if the weather for the day is rain then you don't need to water them yourself as the rain will do that. Unfortunately, there is no way to predict what the weather will be for the next day, so you will have to wait until the morning to fully plan your day.