I have a D810 and am impressed with the times that I had to shoot it at high iso (8000-10000), the shots had some visible noise on the raws easily handled with photoshop noise reduction. Have not as yet started to use it underwater, my first trip with it is comming up soon. I would go with the 810 over the 750 as it is a pro body, with enougfh MP after a dx crop to give you a great image. At 1.2 crop it is a 24mp camera that shoots at 6fps. You will appreciate the 36mp full frame on wide angle shots.
If budget is an issue the D800E will also yield spetacular results, but it does not focus as well as the 810 or have as good an iso response.