These pressures were most likely measured at the blow out preventer (BOP) on the ocean floor. I would discount water depth as having anything to do with the SIP (shut-in pressure).
I assume these pressures are from a DST. The standing oil in the pipe (after all gas is vented off) will add appx .4 lb per ft. Salt water in the pipe (if any) will add appx .5 lb per ft. Nothing to do with depth of sea water. Pup or someone else could explain this better.
It doesn`t make any differance how deep the water is where the well head is on the ocaen floor,it doesn`t effect the persure in the caseing of the well.the well presure depends by the depth of the well and how large the reseiver (oil or gas pool)is.