The speed and violence of the tectonic processes are different.
Divergence is slower and somewhat gentler, and there is less restricting pressure to oppose the upwelling magma (Water may be heavy, but it IS fluid.) The fluidity and cold temperature of the ocean absorb and dissipate the violence of forcing magma out of the mantle and the encounter between great heat and cold waters.
Convergence may combine tectonic movement from multiple vectors, different types of plates may force an oceanic plate under a continental plate and the collision speed will determine the subduction angle and depth. The ejection of material from great depth, through an overriding solid plate, requires great force and pressure. Where hot material encounters underground waters, resultant steam may be superheated and compressed.
Note, however, that volcanic eruptions resulting from deep underwater hot spots may be the most violent of all such events, belying the question’s premise.
After the volcano eruption, evaporation over ocean becomes lower, because it is largely determined by the ocean skin temperature change. The quick response of evaporation to the surface cooling and the delayed response of river runoff to the associated lower precipitation lead to an increased sea level.
