To get it, right click on Atlantic Fleet in the Steam client and select properties. Then under the betas tab select beta.

Here's whats in it:
- fixed low quality ocean shader compatibility
- decreased sensitivity of horizontal slider keys: j k
- improved responsiveness of vertical slider keys: u i
- corrected Slider (coarse) typo
- added full 22:9 aspect ratio support
- corrected German torpedo speed/ranges
- added audio to tiny splashes