Assuming of course that I can also get an appropriate interchangeable lens system camera and underwater housing at a reasonable price...

Mind though that actual lens specifications and pricing have not yet been divulged. Not to mention that capturing & recording 3D footage is just one side of our normal way of seeing things, with processing & editing and playing & viewing, other, as yet to be clarified, facets.
UPDATE: Found a very informative article about stereoscopic 3D:
→ "A Beginner’s Guide to Shooting Stereoscopic 3D" by Tim Dashwood

UPDATE: Panasonic Lumix G 3D lens specification: 12.5mm (65mm*) F12 (fixed), stereo-base: 10mm, minimum-focus-distance: 0.6m.
UPDATE: Panasonic's GF2 & GH2 cameras can only record 3D stills with this lens, NOT 3D videos :o( Older MFT cameras won't support it at all.