While you were swimming/floating at the top of the water while sprinting at the same time ( Sprint button for me is 'F' ) at one moment it start running and then suddenly stop, then start, then stop, this occurs very fast.  and creates that buggy felling like sprint key is broken ( similar like sprinting is interrupted outside of the water ) 

So I know there're uptades that includes alot of,specific, Water updates , but I suggest to work at least something about it , hopefully repearing it . 

This is just an example how it looks on the ground : https://www.youtube.com/watch?v=rHzUzhQMrhE  similar stuff is happening at the water surface!!!