The undersea volcano, El Hierro, in the Canary Islands has been in an eruptive phase since October 2011. The volcano is thought to vent approximately 70 meters below the surface. Surface events vary from jacuzzi-like roiling of turbid water to vigorous upwelling rising many meters above the ocean surface.
The blog Eruptions over on Wired is keeping close tabs on this event as it unwinds.
It is worth pointing out that a volcanic occurrence like this, in addition to land-form building, can also be viewed as a geochemical event. Subsurface eruption of magma comprises the extrusion of fluid rock as well as the injection of gases and solubles into seawater. In the process, water is flashed to steam which adds momentum to the upward convection of the water column from the eruption zone. This causes mixing to occur, tempering the water temperature and dispersing dissolved materials into the currents.