Air resistance?

Air resistance does not play a part when you are weighing in water.

When an object is immersed in water, it displaces water equal to its volume. Water in turn tries to push it up. This is called buoyancy. Hence, the meter outside the water registers the net effect of the two forces;
1) Weight of the object
2) Buoyant Force acting on the object

These two forces act in opposite directions to each other. If they are equal in mangnitude, they cancel out each other and the meter shows zero.