Running and biking experiences make you think that more work is the way to get better. But swimming has more in common with such skill sports as tennis and skiing.
Water robs the swimmer of energy and efficiency.
With every stroke you take, the water is applying the brakes, trying to drag you to a halt, stealing energy from you. Nine of those calories went directly into getting you off the ground. One was lost to the slight inefficiency of the muscular exertion. Water is a frustrating medium for the person trying to move through it, and the nature of that medium makes the swimming puzzle far more difficult to solve than running and biking. Let's compare it with running.