It really depends, not all dirt/water is the same. Sometimes we get lucky and it lands in a very nice spot and in an ideal condition to begin with. Sometimes it's rather violent, and the soil conditions extremely harsh. There are some regions here in SoCal that were prehistoricly underwater in the ocean, and they are now many miles from the nearest shoreline. However the soil is so saltine that with the help of 50-years of southern-california rainfall a V-12 engine lawndarted into it will disintigrate into half.
Without some soils reports, you have no idea what condition they're in, but with one you got a pretty positive idea. In regards to the infamous P-38s bulldozered into ditches at the end of hte war, we'd need to go down the list of location by location, but given they're probabley all sub-tropic volcanic or coral islands in the south pacific - odds aren't good that there is even some metal worth scrap.
Just saying, and trying to educate both sides on this arguement.