we need 2.5*10^9 joules to boil 1 cubic metre of water, assuming the water is already at 100 degrees. over our entire volume of water, that is a total power of about 10^21 watts. that is about 10^-6 of of the power of the sun, not bad, but we currently get only about 10^-15 of that energy, so it would require a brightening of the sun by about 10^11 times.
slap on some sunscreen.
of course it wouldn't all be boiled like this, I am just providing it as an example, since all the water from st helens was in the form of vapour. but I could always do some pressure calculations if you like to estimate the pressure at which the water must have been jettisoned out. they will be big numbers though, and probably higher than the pressure downwards due to gravity, which would mean that the water would be launched into space (never hitting the earth and causing a flood) or just ripping the planet apart.
as you can see though, even with 10000 volcanoes of 10 square kilometers each erupting nothing but liquid water for 40 solid days, you have a major problem on your hands, since there is no evidence. I have not taken precipitation into account in this, but even if you were to factor rain in, you aren't going to get rid of several orders of magnitude from the volcanoes.
just to quote, some extreme rainfall records include nearly 1.2 m of rain in one day during a typhoon at Baguio, Philippines; 0.3 m within one hour during a thunderstorm at Holt, Missouri; and 0.063 m in over a 5-minute period at Portobelo, Panama. these are real extremes, and you would need some really really odd weather to be able to maintain those sorts of conditions for 40 days.
