I have an electronic water level.
It has a sensor on one end, you fasten it to the wall or stake or wherever your reference line is. It has a reference line on it.
The other end is just an open tube. You take the open tube with you, to the point you want level with the reference.
As you move the tube upwards, the water level follows you up. When the water level reaches the reference point, it completes an electrical circuit inside the sensor. Then it beeps to let you know you've found level. I usually double check it and it's always dead on.
It's a one man water level. Great for all kinds of things.
amazon.com - water level