What about shallow waters? I lost a UFO to the Great Salt Lake in Utah, USA, which, trust me, is not so deep as to prevent some sort of recovery. If the aliens drown, then the UFO can still be recovered (Im sure that it would still be sticking up out of the water, no matter how it entered)
And there were several older maps that gave long range weapons the advantage, like the dam map.
BTW: What happened to those maps?