The issue with chemical weapons is that they are very reactive. Mustard gas would react with the organics floating around in the ocean very quickly.
As for the hydrolysis product of mustard gas, thiodiethanol, it's not very toxic. The MSDS claims acute toxicity in rates starts around 6.5g/kg. So a human would need to consume ~500g in order to see toxicity.
Also, thiodiethanol is quite reactive itself. It would likely be oxidized to the carboxylic acid. Wouldn't surprise me if bacteria could metabolize it.
Yeah, given the existence of life at deep-sea sulfuric vents, I wouldn't be surprised if the newly evolved bioorganisms were colonizing the containers at their leakage points.
The point is the ocean is known to contain bacteria which can metabolize chemical sources in the form of sulfur compounds. There's a chance that any slowly leaking CS munitions in the Baltic Sea could host bacterial colonies which break down these compounds.
Maybe much less potent in killing instantly (ie in combat use), but what about long-term low-dosage exposure?
> possibly going up the food chain (does that happen?)
Bio-accumulation in general is very real thing, and imho the most worrisome aspect of this situation.