Abstract　 　　 

　 Herschel extragalactic wide surveys which have observed about 1000 deg2, have discovered hundreds of thousands of high-z dusty sub-millimeter galaxies, and hundreds of high-z sub-millimeter strongly lensed galaxies. Gravitational magnification there allows a gain by an order of magnitude in sensitivity in studying sub-millimeter galaxies at high redshift (z~1-5). I will first summarize the current strategy for identifying, characterizing and following up these sources. I will mainly highlight strongest lenses, highest redshift sources ( z~ 4-6), lensed AGN and possible proto-clusters detected by Planck and Herschel. The easy detection of H2O lines redshifted into millimetre atmospheric windows is a good illustration. I will update the results of our program of millimeter detections of H2O lines at high-z. Observing a sample of 14 strong lenses from the H-ATLAS survey with IRAM interferometer, we have shown that J=2 H2O lines are easily detectable in all of them. H2O lines provide thus an important diagnosis in extreme starburst and possibly AGN conditions of warm gas of these galaxies, especially those with LIR ~10^13 Lo (SFR>1000 Mo/yr), which have no local equivalents but are abundant at z ~ 2-4. The weaker emission obtained from the first results of the extension of the observations at higher excitation J=3 levels might indicate that the J=2 emitting medium is more extended than the cores of local ULIRGs. However, modeling H2O emission remains difficult since complex processes take place such as IR radiation transfer, shocks, cosmic rays, AGN feedback and XDR chemistry, outflows, etc. Finally, I will evoke the rich prospects with ALMA and other facilities for various possible extensions of such studies.