California

Seawater is alkaline (pH average ~8.1), meaning pOH is about 5.9. Dissolving 40g of sodium hydroxide (a strong, corrosive alkali) in 18,000 liters (less than 600 gallons) of water would give a solution less basic than that. With acidity and basicity, it's the concentration that makes it dangerous, and high dilution easily renders it harmless.

What the evidence shows, mostly, is that these barrels were well sealed, and that the alkali has been leaking out very slowly. Possibly, the best thing to do would be to poke a few small holes in each barrel, and let the alkali diffuse out slowly so that the conc'n drops off to something similar to the rest of the ocean around it -- quite harmlessly. The statement that the contents "will take thousands of years to break down" seems rather sloppy. Alkali is always present in the ocean, and it doesn't "break down", though it can be reversibly neutralized by acid. Unless ....

The BIG question is, why was this disposed of as "waste" ? Because it was used in some industrial process, and has other, quite possibly toxic, contaminants ? If so, that's a much larger concern than the simple presence of alkali. Alkali is safe when diluted, unlike many toxins. There is a chance that it was simply surplus to needs after wartime production shut down. That would be the best-case scenario, but more testing of the contents would be needed to establish that.