When you jump in the water and press the Sprint key, or double tap the Run key, it changes your hitbox to the swimming hitbox, but the controls and speed remain the same as walking on land. In order to begin swimming, you must sink to the bottom and touch the ground, stand up underwater, and then begin swimming. 

Possibly related: in previous versions, when you were flying with elytra or riding a boat, pressing the Sprint key or double tapping the Run key would make you go faster, but that no longer occurs.