We report a search for H_2_O megamasers in 274 SDSS type-2 active galactic nuclei (AGNs; 0.3<z<0.83), half of which can be classified as type-2 quasi-stellar objects (QSOs) from their [OIII]5007 luminosity, using the Robert C. Byrd Green Bank Telescope (GBT) and the Effelsberg 100m radio telescope. Apart from the detection of the extremely luminous water vapor megamaser SDSS J080430.99+360718.1, already reported by Barvainis & Antonucci (2005ApJ...628...89B), we do not find any additional line emission. This high rate of nondetections is compared to the water maser luminosity function created from the 78 water maser galaxies known to date and its extrapolation toward 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 (tables 4 and 5), we list 173 additional objects (mainly radio galaxies, but also QSOs and galaxies) that were observed with the GBT, the Effelsberg 100m radio telescope, or Arecibo Observatory without leading to the detection of water maser emission.