How about the Nemrod Metzeler S.A. Siluro-S a 6x6 underwater camera made of Novodur plastic in the 1960s, the Leicashop in Vienna has one for sale
the camera looks pretty simple according to camerapedia one shutterspeed and a fixed focus lens and requires a flash
A 35mm UW camera is a better choice and the older Nikonos or Sea & Sea cameras can be bought quiet cheap.