These are chemical weapons. Just like they're destructive to humans, they're destructive to everything that lives in the sea. If you blow them up remotely you just let everything that was inside loose.

It depends on what is worse, a slow leakage that can poison the life in the immediate vicinity for decades, or a quick explosion that is immediately lethal to life in a large volume but is quickly diluted.

Also, I don't believe that arsenic-containing chemical agents were used in WWI. Lewisite production started in 1918, too late for it to enter the war. And other chemical weapon agents are not persistent polluters.

But if nothing is done, then something like that will happen at some point. It may be an explosion, or it might leak out over a period of time. No matter what it will cause major damage, and we wont know that is has happened until it is pretty much over.

If they discover a bomb like this leaking most dangerous chemicals into the sea they don't have any way of addressing it I guess.

Slowly leaking out over a period of time is arguably the lesser of possible evils if there is no safe way to dispose of them otherwise. On a long enough timescale water circulation can dilute the effects, the slower it leaks the better. Of course this is worse than being able to take all of it out to begin with, but if that's not an option we have, then that's better than suddenly releasing all of it at once.