Deep-blue sea

[ReyeR]

It's possible the camera's flash being ON or OFF has some influence on the WB when the camera processes the final image.
Leave the WB on Auto and use Photoshop to control the colour balance (RGB).

Re: MAY
Yeah, I found that to be amusing too. It should either work or not work

 

[ReyeR]

Sounds like I MAY need to go and buy a better camera

. MAYbe.
FWIW I have the same problem with the C5050 in M mode. In the same dive, same aperture and shutterspeed and external strobe ON I get water ranging from Hawaii blue to pea soup green. It just depends on what the camera 'sees' as white in the picture and uses to calculate the white balance. This is one of the reasons I have started shooting more and more in RAW format.
However, what M will give you is better control over the balance between ambient light and strobe light. Following Karl's (kdietz) procedure you can use the shutter speed get backgrounds that go from blue to black. And use the aperture and strobe output to control the foreground exposure.

 

[Tobagoman]

Just a quick tweak with Photoshop curves with the 'greenish' picture of yours:

How exactly did you do that? It looks great compared to the original.