There is one area of marine submersion that could possibly result in a repairable find after 70 years of that marine submersion - and that is, where the item is buried in fine sediment that encapsulates the submerged item. The Mary Rose is a good example of this.
In addition, if the particular submersion point was close to the shoreline, where fresh estuarine waters regularly enter the sea, then the salt content of the water is regularly lowered or diluted, thus reducing the corrosion effect of the sea.
I spoke to an old veteran many years ago, who was an engineer in WW2, and he was a Milne Bay veteran.
He related a story where he was instructed to drive a near-new Caterpillar bulldozer ashore from a landing barge, as the Milne Bay invasion was launched.
He said he was informed the ramp would be dropped as soon as the barge bottomed out, and the water would only be chest deep at most, and he was instructed to drive the bulldozer ashore and commence clearing areas for the landing of following armaments and war materiel.
He told me the ramp dropped, he gunned the dozer down the ramp, and off the end - and the dozer promptly went out of sight! - into soft, estuarine silt, that he reckoned was at least about 20 feet (6M) deep!
He baled out and struggled to get ashore as the silt provided poor footing - let alone a surface suitable for landing other heavy items such as weapons, tanks and trucks.
He informed me how he abandoned trying to get ashore and climbed back onto the barge, and was then quite vocal to the CO, about how "the water was chest deep, alright!! - but you forgot to tell me about the 20 feet of silt!!"
Apparently, there was some hurried decision making that entailed the barges reversing and moving to another landing point that provided more suitable underwater footing for the landing.
Of course, all this was due to inadequate intelligence, mapping, and other vital geographical information - a large feature of the WW2 New Guinea campaigns.
I often wonder how that bulldozer has survived as regards preservation, totally encapsulated in that fine estuarine silt. I often wonder what else is buried in similar fine silt, in many other campaign zones, where it was encountered.
It's entirely possible there may be valuable historic aircraft buried in fine estuarine silt and fairly well preserved - a la the Mary Rose - if they were wrecked in estuarine locations close to shore.
As regards the instructions issued to the ships crews dumping surplus WW2 materiel, vehicles, armaments and aircraft - they were told to dump the items in water depth that ensured that any recovery attempts by entrepreneurial types would be thwarted.