  We report a search for H2O megamasers in 274 SDSS type-2 AGNs (0.3 < z <
0.83), half of which can be classified as type-2 QSOs from their [OIII] 5007
luminosity, using the Robert C. Byrd Green Bank Telescope (GBT) and the
Effelsberg 100-m radio telescope. Apart from the detection of the extremely
luminous water vapor megamaser SDSS J080430.99+360718.1, already reported by
Barvainis & Antonucci (2005), we do not find any additional line emission. This
high rate of non-detections is compared to the water maser luminosity function
created from the 78 water maser galaxies known to date and its extrapolation
towards the higher luminosities of "gigamasers" that we would have been able to
detect given the sensitivity of our survey. The properties of the known water
masers are summarized and discussed with respect to the nature of high-z type-2
AGNs and megamasers in general. In the appendix, we list 173 additional objects
(mainly radio galaxies, but also QSOs and galaxies) that were observed with the
GBT, the Effelsberg 100-m radio telescope, or Arecibo Observatory without
leading to the detection of water maser emission.
