There's some more info here which might be of interest:
http://www.exercisetigerslapton.org/
... and here:
http://www.submerged.co.uk/slapton.php
To let the thread drift a bit, the wreck of the SS Richard Montgomery lies in shallow water in the Thames Estuary. It was carrying approximately 1,500 tons of explosive when it ran aground on a sandbank and the hull cracked open. Opinion seems to be divided on the likely effects of the cargo exploding, but the authorities are erring on the side of caution.
http://en.wikipedia.org/wiki/SS_Richard_Montgomery