Fill a medium saucepan with a cup of water; bring to a boil. In a separate bowl, mix the egg yolks with the lemon juice, water, and mustard. Place the bowl over the saucepan of boiling water, taking care that the water does not touch the bottom of the bowl. Keep mixing until sauce begins thickening, at which point you should add the melted butter, stirring constantly, bringing the sauce to a thick creaminess. If at any point the sauce appears too thick, add a bit of water, being careful not to let it get too runny. Set aside somewhere it can stay warm while you work on the rest of the dish.