I have found this to be quite a problem this year and the only practical way of getting it out is with a mechanical pump. I have made myself a rig using my oil transfer pump. Basically on the end of my suction hose I have about 2m of 22mm copper pipe with a 90 degree bend on the end. I connect my suction hose onto this with a jubilee clip and dangle the delivery hose into a 200l open topped barrel. What I then do is suck out and use the copper pipe like a vacuum cleaner, sucking the water from the bottom of the tank, moving the end of the copper pipe over the base of the tank. You will get a lot of oil with it so wait until the barrel contents settle out and you can pump the oil back into the tank and leave the water layer. Remember that the water is contaminated so you need to dispose of properly and charge the customer accordingly.
I find that the usual source of water ingress is from the filling point - over the years with the fuel companies constantly screwing and unscrewing their hoses on it, it tends to work loose allowing rainwater to get around the seal. I re-do them with silicone sealant.