Sam,

Unless the explosives are encased in brass, a possibility if the hold contained munitions, or the TNT were held in watertight containers, I don't think you have much to worry about.

Not saying that some WW I and II mines are no longer still deadly, they are. And that's not getting into the unexploded WW I mines in the Somme, and one detonated when struck by lightning in 1955!

Here's quote from a study done on TNT in the North Sea:

From the study:

"Overall, this laboratory study showed that the attenuation of TNT is slow (half-life in seawater ca.1900 days; half-life sand sediment <700 days; half life in clay sediment 130 days) under conditions commonly encountered in coastal waters of the North Sea."

https://www.researchgate.net/publication/44583383_Attenuation_of_TNT_in_seawater_microcosms

Lastly, typo alert...the invasion of France on the beaches of Normandy was in 1944.