[theDentist]

Wouldn’t we have alpha detectors??? :-)

[BwanaNdege]

Alpha particles are easily stopped by a sheet of paper.

IOW, any alpha particles emitted by the nuclear material in the bomb will be stopped by the explosive wrapping or metal case or the paint layer.

[GonzoGOP]

Water is a fantastic alpha blocker. That's why they store nuclear waste at the bottom of pools of water. Unless you got right on top of it you wouldn't be able to detect a thing. You would actually have a better chance looking for the rusting metal case using a magnetometer. Of course you would also pick up every shipwreck and old fridge on the bottom, but it would be a start.

The fissionable materials have degraded so much in 50 years that there is no way to send it critical now. Nuclear weapons have a very definite "Use By" date. So the water contains the radiation and it no longer has any use as a weapon. After this many years the worst thing you could do is bring it up.

[arthurus]

Alpha particles don’t go very far in water.