That's not quite true. The crops appear on a fixed schedule, but if your villagers harvest them quickly you will have to wait longer for the crops to appear than if your villagers harvest them slowly. If the crops are not all harvested by the time the next crops are scheduled, any crops remaining unharvested will be lost.

Farmers gain skill when engaged in any farming activity, including watering the crops. Master farmers are an important asset to the game, so letting them water the crops is not a waste of time.