Mount Pagan in the Mariana Islands erupting on April 15, 2009. Image courtesy of the NASA Earth Observatory.
Up until fairly recently, a lot of volcanoes got away with erupting without anyone knowing the better. However, with all the "eyes in the sky" we have these days, even remote oceanic volcanoes get caught in the act of erupting. Case in point is this image (above) of Mt. Pagan in the Mariana Islands erupting. It was captured by the MODIS satellite yesterday and it shows a health ash plume emanating from the volcano. Pagan is an oceanic stratovolcano on the sparsely populated island of the same name and it really two volcanoes (overflight video on this link) bridged by land. The last known eruption was in 2006, but most of the historic eruptions are small (VEI an 18-20 km / ~60,000 foot ash column, so it was definitely significant. It is good to know that even in places where some minor events might go unnoticed, there are resources that help us keep track of many of the potential volcanic threats on the planet.
In the wikipedia article on the 1815 Mt Tambora eruption there is a graph of sulfate concentrations from a Greenland ice-core. That core reveals a unknown eruption (AFAIK, IANAV) occurring sometime before 1810 that deposited almost as much sulfate as Mt Tambora did around 6 years later.
The Pagan 1981 eruption was almost a disaster; despite several months of quakes, there was no evacuation before the eruption. In fact the eruption started while the island's radio operator was reporting on that morning's earthquakes. By the time a plane reached Pagan, heavy ashfall from an essentially Plinian eruption column prevented it landing. The islanders were evacuated by ship next day..after the paroxysmal phase had ended. (information from the GVP website 'monthly reports' for Pagan)
There are many more volcanoes underwater than above water.
These underwater volcanoes aren't going to blast ash into the stratosphere. But if these underwater volcanoes erupt Sulphur Dioxide, will it eventually work its way into the atmosphere? I would guess not because I suppose it would dissolve in the water and not go into the air. But that is just a hunch - I don't know it for sure.
Also there has been some debate about whether volcanoes under Antarctica could speed up the ice flow to the ocean. Some say a a degree warming in the Antarctic air can cause the ice to move faster, but, that a volcano under the ice would have no affect.
Scientists have to keep open minds and consider many variables.
Here is an article where some scientists recognized that a Greenland hotspot was making some ice in Greenland move faster.
Your hunch is correct. Aqueous solutions of Sulphur Dioxide, which sometimes are referred to as Sulphurous acid.
Current report from the USGS CNMI site:
Note that it is a weekly update.