Abstract　  

  As confirmed by Herschel, submillimeter lines of H2O are, just after CO, among the strongest submillimeter molecular lines in local and high-z ultra-luminous infrared galaxies (ULIRGs). They provide a completely different diagnosis than CO of their dense, warm cores because of the large electric dipole of H2O, the high water abundance and their sensitivity to infrared excitation. I will report on the progress of high-z H2O line studies in strongly lensed high-z Herschel galaxies from the H-ATLAS survey. High-z H2O detections with IRAM/PdBI will be presented in 15 objects and compared with Herschel detections in local ULIRGs. We have shown that H2O is easily detectable at all redshifts. The H2O line luminosity increases as LIR ~1.2, and highexcitation lines (Eup/k ~ 300K) remain strong in the majority of the sources. H2O lines provide thus an important diagnosis in extreme starbust (and possibly AGN) conditions of warm, dense cores/clumps of these galaxies which are often more extreme than local equivalents and are abundant at z ~ 2-4. Modeling H2O rotational excitation allows us to constrain physical parameters there such as warm dust opacity and temperature and H2O abundance. Extensions to observations of related molecular lines such as H2O+ will be also presented.