The one issue I had when shooting underwater was with the new accessory flash. I found on occasion it would disconnect, possibly it was slightly jarred from bouncing on the little panga, and I would be unable to use my flash, despite positive test shots prior to getting on the boat. I have never had this happen with previous OM-D cameras, and was unable to get it to work without opening the housing and jiggling the flash attachment. Out of 19 dives on the trip it failed on 2 of them.
Olympus EM-1 MkIII = EM-1X in small body
Olympus EM-5 MkIII = EM-1 Mark II in small body
Might get some good prices on the EM-1 MkII now the replacement is out - if it's close the 1-mkII would be a better option than the 5-MkIII if only for the much bigger battery, the better housing options have been covered already - the 1-MkII housing is really quite nice to use with the shutter and back button focus setup.