Hello,
some years ago underwater casings were sold under the name "EWA Marine". I think these sealed plastic bags were not for deep water. Specialized underwater cases for such old cameras are maybe difficult to find. It could be easier and maybe cheaper to buy a used NIKONOS.
UK-Germany used to make a really nice one for the FE2/FM2 with MD-12 motordrive; v.good, but any housings really need a magnifying finder (which I added myself on mine) added in order to be able to see the whole finder image through a dive mask.
...One of the reasons I went to an F4 housing in conjunction with the DA-20 finder.