If you have updated LR recently to v14.3.1 you might have discovered a new Mask, Landscape, in the Develop Module.

I've just played around with it a bit this evening and have found it works really well for many u/w images. It even identifies the water as, err, water. It seems to be able to differentiate divers, water and corals. I found I could create a number of masks which allowed me to make adjustments to the drivers face, to the corals and to remove backscatter by masking the water.

Definitely worth a try.

Thanks for the tip Tim. I just tried it on an over/under image and it worked a treat on both halves!