There's a lot down there, people have been dumping chemicals weapons in the ocean for at least a hundred years. Once it sinks you can't see it anymore so that means it's gone forever and you don't have to think about it.
There are a lot of old munitions dumps in the ocean and they don't always "stay down there":
The WW2 bombs dumped off western Scotland washing up on beaches
According to a letter sent by the MoD in June to researchers at the University of Liverpool, the MoD dispatched vast amounts of old weapons to Beaufort’s Dyke.
The ministry dumped some 14,000 tons of 5-inch artillery rockets filled with poisonous phosgene gas in the trench between July and October 1945. Over the following three years, it consigned 135,000 tons of conventional munitions there, and every year “into the late 1950s” another 20,000 tons ended up in the dyke.
https://nonproliferation.org/chemical-weapon-munitions-dumpe...