As bb2k says - water is a great lubricant. If you're using a water pump, it's designed to have
water running through it. You don't need to add additional lubricant. If you are using something like an Iwaki, which has a "water bearing" running a fluid with a different viscosity may cause early failure. You are
not going to get longer pump rotor bearing life by adding a lubricant. Slick50 doesn't help your car engine either
...somebody had to say this a bit more directly...