This relates to MC-75279, however that one was fixed in a previous update.
A similar issue is happening when sprinting & "flying" in creative mode underwater.
The cape is doing its usual animation, which doesn't really fit the environment.
See this comment by @unknown for a summary of what causes this while flying.
To fix this, the cape renderer would have to make a difference for when the player is in the swimming animation and change the f2 value to keep the cape down to the player's body.
This seems to be fixed as of 18w22b.
Sprinting in the swimming animation is no longer possible, therefore resolving the graphical issue. (See screenshot)