It is not in LR 6. It's only available if you have the subscription based LR CC 2015.
Thank you, it could have been of use in correcting underwater video.
You may be able to correct your under water pictures using the contrast parameters. That is what I used before the dehaze tool. You need however to adapt other parameters too like the saturation, lightness etc.
You may also subscribe and use the dehaze tool of LR CC. To bad, that you didn't ask the question before...