Do you have a strobe for overwater use? If so, you can test optical ttl also with a "normal" strobe and try to figure out if the problem might be camera setting or the strobe.
Maybe you can inspect the housing that the "window" for the optical cable is clean. I don't think that that will be the problem, but just to be sure.
Also it could be possible you have to use other optical cables. I think I remember some statements that the new version of the D2 has a different sensor, so the optical cables that worked with the old version might not work with the new one.
I have the old version of the Sea&Sea YS D2, so you could test again if the old version would still shoot in ttl-mode, just to make shure nothing else has changed (camera settings), but I live in the south of Germany.
Also, it seems strange, but the Nikon D750 is not listed in the compatibility chart for the D2: