When the player is holding spacebar and bobbing at the surface of the water, there is a moment right at the top of the "bob" when the game doesn't register the player as swimming. Commands will still find the player to be within the water block at this point, it's just the swimming check that doesn't. This bug has a number of effects:

• The swimming statistic will not register the distance moved when you are at the top of your "bob". If you swim at a constant speed, the swimOneCm statistic will increase at a varying speed.

• The levitation effect normally doesn't work in water. However when the player reaches the top of their "bob", it will suddenly take effect, pulling them out of the water.

• The player's hunger will decrease very slightly more slowly than it should, since it doesn't decrease when the game doesn't see the player as swimming. (I haven't bothered testing this though, as it would be an extremely small difference.)

• It's possible that this bug is what causes sprinting and speed potions to take partial effect while swimming. I haven't tested this though.

This bug also occurs in lava, although it's less visible since stat.swimOneCm doesn't work in lava (MC-101297).