Abstract Herschel extragalactic wide surveys which have observed about 1000 deg2, are discovering hundreds of thousands of high-z dusty submillimeter galaxies, and hundreds of high-z submillimeter strongly lensed galaxies. Gravitational magnification there allows a gain by an order of magnitude in sensitivity in studying submillimeter galaxies at high redshift (z~1-5). I will first summarize the main results of these submm surveys, and the strategy for identifying, characterizing and following up these sources. I will briefly highlight mainly strongest lenses, highest redshift (z ~ 4-6) sources, and possible proto-clusters detected by Planck and Herschel.
Then I will focus on H2O line studies in these lenses. After evoking Herschel results on H2O emission in local Ultra-Luminous Infrared Galaxies (ULIRGs, SFR > 100 Mo/yr), H2O detections will be presented on similar objects at high-z. Observing a sample of lenses from the H-ATLAS survey, we have just shown that H2O is detectable in practically all Herschel strongly lensed galaxies with the current sensitivity of IRAM-PdBI. We show that the H2O line luminosity increases as LIR1.4-1.5. It thus reaches pretty high values when LIR ~10^13 Lo (SFR > 1000 Mo/yr), which have no local equivalents but are abundant at z ~ 2-4. H2O lines provide thus an important diagnosis in extreme starbust and possibly AGN conditions of warm, dense cores of these galaxies. Other new lines such as OH, OH+, H2O+, HF will complement this diagnosis. However, modelling the infrared dominated excitation of H2O and other lines remains difficult, especially in the context of ULIRG cores where complex processes take place such as IR radiation transfer, shocks, cosmic rays, AGN feedback and XDR chemistry, outflows, etc. finally, I will briefly evoke various possible extensions of such studies, especially with ALMA.
