The Savannah nuclear bomb is certainly a concern!! Although the military claims there is no danger of a thermonuclear detonation, the idea of an undisclosed amount of Uranium and Plutonium laying at the bottom of a shallow water bay near a US city is troubling. I suppose they may eventually recover it, once the weapons case corrodes away and exposes the radioactive materials to water.
According to the MSDS sheets, the Mustard gas bombs in this case are phosgene which is deadly in concentrate at atmospheric pressure. It does decompose when exposed to water. Unfortunately, the reaction releases Hydrogen Chloride and Carbon Monoxide also toxic gases. Underwater they're probably fine, exposed to the atmosphere on the deck of a shrimp boat or work boat would be another story.