Manual white balance has the annoying need to recalibrate when you change depth or lighting which can sometimes be inconvenient depending on the dive.
You can pull a lot more in the way of highlights out of the RAW than JPGs and other things aren't lost in compression or the cameras sharpening. Some are better than others in their rendering of jpgs but at the end of the day there is more scope for rescue and alteration from a RAW image. Also useful is to increase exposure from an underexposed shot which i tend to do a lot underwater due to locking a shutter speed and not owning a strobe. JPG seems fairly bad at that.
The hack can be found here:
CHDK - CHDK Wiki
As well as RAW it introduces other features like custom iso, shutter speeds, f-stops, increase flash sync speed and a lot more features generally only found (or unlocked) on more expensive.