I don't know if this really is a bug, or is intended, but whenever you're both on a block with 1.5 block height and in water, you will find yourself unable to jump unless touching another block. This was first experienced by myself in my survival world, then my creative world to test more situations. Another strange occurance is while in this "unjumping" state, if you push against the face of any block, it automatically makes you jump.