Question

why is it easier to lift a mass inside water than in air

Answer 1

because air exerts pressure

Answer 2

Andrew is flat wrong. Of course it is easier to lift objects in the water than in air. Objects in water actually weigh the same as out of the water, but are easier to lift. The force needed to lift an object in water is the weight of the object minus the weight of the amount of water the object displaces.

So a small rock will not be very much easier to lift either way as it is heavy is comparison to the amount of water it displaces.

However a human is mostly water and large in relation to the amount of water it would displace. The force needed to lift a 150 pound person is quite small. Depending on the person they might just float to the top.