I'll add a bit to the question. As most (or all) people should realize, water vapor is a green house gas. The implication in your question is that there is a point where positive feedback from additional water vapor in the atmosphere may lead to a situation where a LOT of water vapor enters the atmosphere, leading to a temperature runaway whereby the surface temperature of the earth is 100 deg C or higher.
This scenario is actually impossible. Water vapor is not only a green house gas, but it is an EXCELLENT heat transfer medium, such that on a mass basis, water has a heat of vaporization that is approximately 4 times that of most other liquids (e.g., hydrocarbons). Given this fact, atmospheric convection currents would carry any additional water vapor high into the atmosphere, where it would radiate energy to outer space, condense, and fall as rain, long before the concentration had a chance to build up to the "boiling seas" situation.
The only way that I can imagine the "boiling seas" scenario would necessarily require a much more dense atmosphere, where the lapse rate was high enough to cause extreme temperatures at ground level. That much gaseous nitrogen and gaseous oxygen currently doesn't exist on earth, and it's non-intuitive how that much extra atmosphere could be generated. In the event that such an atmosphere could be generated, temperatures would rise to the point where the heat transfer from condensing water vapor would balance the radiant heat arriving from the sun, resulting in an "equilibrium" temperature that would meet the conditions of your question. Even then, the oceans wouldn't boil away, because atmospheric pressure would be much higher than it currently is, and the boiling temperature of water would be much higher at sea level as a result.
For people that still have doubts, note that the earth has been around for a LONG time, and experienced various and wide-ranging environmental conditions. There is a lot of empirical evidence that says that the earth has never been in the state described in the question, which strongly implies that there is no net positive feedback loop with respect to water vapor. Positive feedback is inherently unstable, and if it existed, we certainly would have evidence of that fact by now.