NB: not my specialty, so I've likely missed other effects. Searching scholarly literature for "SO2 impacts on marine life" and "NO2 impacts on marine life" could be illuminating.

Sulfur dioxide in particular produces sulfuric acid. Small changes to pH and specific acid/base concentrations of seawater, even if limited to a narrow vertical column, could have large impacts. Say, non floating plankton, especially primary producers.

There's also the potential for positive "seeding" effects by introducing limiting nutrients to water, though my understanding is that iron and carbon are more likely to play these roles.