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 https://www.leicashop.com/vintage/ne...os-p-5105.html
the camera looks pretty simple according to camerapedia one shutterspeed and a fixed focus lens and requires a flash http://medfmt.8k.com/mf/nemrod.html
A 35mm UW camera is a better choice and the older Nikonos or Sea & Sea cameras can be bought quiet cheap.

Dominik