Yep, from the site it seems the MMII accessories will not work on the new MMIII - not surprising as the basic lens is different. But then you get a 20mm wide angle straight off, and the 35mm angle lens is not terribly useful underwater. Also the new camera is rated to 60m instead of 45m.
And there is a range of MMIII accessories:
a 12.5mm fisheye conversion lens (89,000 yen)
a 15mm wide angle conversion lens (29,000)
2T Macro 25,000 yen
3.5 T macro 23,000 yen
Multi viewfinders, fish eye viewfiner and lens caddy there too. NB the prices are Sea and Sea's own recommended prices - in Tokyo, you can probably find these cheaper, and no idea what they will become when Sea and Sea US starts selling them.
The MMIII will work with the Sea and Sea YS strobes.
Murat, if you're interested in either the MMIIEX or the MMIII, I'd suggest getting the strobe and playing with the MMII you have to see if you like it first. A Sea and Sea strobe will still work with the newer models, and a range of other cameras too - anything that has either the MMII or the Nikon connector. You could get the YS 90DX, which will also work with digital housings, so if you like u/w photography but not the cost of developing film with 33 fish butts and 3 good shots, your strobe won't be worthless if you switch.
And you can always E-Bay it if you decide u/w photography is not for you.