DOF is a nightmare underwater... My hat would be off for anyone who shot LF underwater just as my hat is off for those 16x20 glass wet plate photographers that ventured into Utah and beyond in the 1850's
36 exposures are not much underwater. I did buy a Nik V 35mm and 20mm with finder in near mint and useable condition for under $500 this summer. That 20mm is needed the 35mm just doesn't quite do the job, in clear water the 20mm seems like a Ziess (or Nikkor... or whatever coated lens you love) out of water.
Sorry to repeat the much more experienced UW poster "MAtt" above. exactly what I was trying to say above.