Never said "water tight" just they were wrapped and preserved for the trip overseas. In THAT condition, they are better protected than just burying the ship sans protection.
Having shipped MANY aircraft and being on the other side removing them from shipment, they can hold up pretty good. Now that isnt burying them for 60+ years agreed but that is what is going to make it a better raise and restore than taking one off the ocean floor. The dirt will act as an insulator controlling damages of time. I am sure the crates are all but gone, and the tar paper is going, so getting them up and checking them now.
I was part of an experiment for the Army in the 80s where we took a UH-1 and AH-1 and buried them. A year later we dug them and and returned them to service. Our group used the sandy loamy soil of North Carolina, another group did the desert. After cleaning and reservicing the major systems we were surprised to note that for the most part the ships were in pretty good condition. That was only one year, and with little preservation, but soil conditions were pretty similar to where the Spits are buried. I would still love to see their conditions. I suspect they are rough but rebuildable.