IMHO, the Sony cameras have never been great at MWB underwater. The MWB software seems to be desperately looking for red. You can fudge it with a pink plastic card but it's still a bit hit and miss.
Rather that stuff-around with MWB while diving, I prefer to be able to quickly get to a WB that approximates the conditions. The typical conditions are:
- Light blue colour cast - shallow, clear tropical water
- Deep blue colour cast - deep, clear tropical water
- Light blue-green colour cast - shallow tropical water with some suspended plankton
- Deep blue-green-green colour cast - deeper tropical water and/or lots of suspended plankton
- Video lights dominate (night, cave, etc.) - use AWB