Occasionally, when you swim underwater, it goes completely black, and you can't see anything. This was present in 1.8.1, and still occurs in 1.9.0. I am well aware this is a duplicate post, but this problem needs to be moved up and patched in 1.9.1 instead of waiting 'til 1.10 or later, as it greatly affects gameplay. Please at least confirm if this is being worked on before marking it as a duplicate, as well as when we might expect it to be fixed.
It is in the internal bug tracker
There is no ETA of when a update for when will be out that would fix this issue.