The hydro-bomb thing was completely ridiculous. Much of the groundwater is above the normal boiling point anyway, with pressure being what keeps it liquid. Hell, the earth's mantle has as much water trapped in hydrated minerals as all of the Earth's oceans. Hell, a lot of volcanic eruptions (and geysers) are caused when pressurized, super-heated water and gases manage to work their way to the earth's surface suddenly.
Regardless, the energy needed to boil all of the groundwater on a planet would basically cook the entire crust and turn it to lava anyway. You wouldn't need the volcanoes to erupt.
A better use of technobabble would have been some sort of dark energy weapon. Just change the cosmological constant to increase the baseline level of inflation of space in the local universe around Qo'nos. Gravity would be canceled out, and everything would begin breaking into smaller and smaller chunks.