Simple alkanedithiols exhibit the same molecular conductance whether measured in air, under vacuum or under liquids of different polarity. Here, we show that the presence of water 'gates' the conductance of a family of oligothiophene-containing molecular wires, and that the longer the oligothiophene, the larger is the effect; for the longest example studied, the molecular conductance is over two orders of magnitude larger in the presence of water, an unprecedented result suggesting that ambient water is a crucial factor to be taken into account when measuring single molecule conductances (SMC), or in the design of future molecular electronic devices. Theoretical investigation of electron transport through the molecules, using the ab initio non-equilibrium Green's function (SMEAGOL) method , shows that water molecules interact with the thiophene rings, shifting the transport resonances enough to increase greatly the SMC of the longer, more conjugated examples.