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The lower viscosity of water reduces friction. Besides, the water can transfer heat way better than oils. So why not use water as a lubricant in wind or hydro turbines, especially on equipment which is operating in the ocean ...
It seems to be a perfect material. It's the most abundant liquid on Earth, and it causes no ecological damage.
The whole point of lubrication is to keep the two surfaces almost apart.
Water has low viscosity in comparison to oil-based lubricants, which means that it forms a thinner film (electrohydrodynamic film thickness) between the two surfaces; the thinner the film, the greater the chance of direct physical contact between the parts.
So, the higher viscosity actually can be useful here. Another thing about water is that its viscosity doesn’t increase by pressure, unlike oil-based lubricants, as usually, the two surfaces are operating under pressure.
Oils are better vibration dampers than water.
As @SolarMike mentioned in the comments, water is extremely corrosive and can also dissolve many destructive elements.
Thanks to the capillary effect, water molecules can penetrate into micro-cracks on/beneath the surface of parts. Once there, due to extreme pressure, those molecules can decompose into hydrogen and oxygen, and build up pressure in the cracks, which leads to crack propagation and failure. This phenomenon is called Hydrogen embrittlement.
Water also provides a very good environment for fungi and other living microorganisms, which decay and changes the PH of the water, usually a shift toward an acidic environment, which in turn also corrodes the steel.
Sea water is about the worst thing in the world to put in a metallic mechanical device. As noted , corrosion would be horrendous ; dissimilar metals, stray currents , etc. Wind turbine gearing is an exceptionally demanding application ; EP ( extreme pressure ) lubes are needed with graphite and/or moly sulfide, etc. I am sure a major problem addressed by wind machine engineers for off shore applications , is sealing the lube systems to keep salt and moisture out. Not the least of problems is that water has little/no lubricity qualities like film strength.
Great question! Water absolutely is used as a lubricant in some power generating systems. For example many Francis style turbines use their feed water as the primary lubricant and coolant of the bottom main bearing. (In the old Loeffel turbines, that bearing was typically made of lignum vitae wood, and could last up to 100 years in near constant use). Good engineering takes advantage of the cheap availability of water whenever applicable.
But water freezes and expands, blowing apart your engines, so it's not suitable for every application. This is enough of an everyday issue that automobiles are designed with "freeze plugs" and block heater ports and typically use toxic and environmentally unfriendly antifreeze chemicals in their liquid coolant systems. If your generator fails in cold weather, you don't want it to freeze solid before you can manage to repair it.
(A freeze plug, also called a Welch plug, is a core hole plug purposely made weak enough that it will fail before the block cracks due to frozen coolant. Core holes are left by the block casting process.)
And although absolutely pure water is an electrical insulator in the laboratory, in real life it's not, because even the slightest amount of contamination makes water an excellent electrical conductor. Oils, on the other hand, typically have consistently low conductivity until they are so loaded with metal contaminants that they have lost lubricity. If your generator has an oil leak, the oil won't provide a conductive path to repair technicians and other system parts.
And finally, as others have already noted, water is very corrosive, especially seawater. Since rust expands to five times the size of the steel it came from, you can't use any of the cheap iron alloys in water, which drives up the cost of materials pretty quickly.
Petroleum oils are a superb lubricant, able to be purified and recycled many times. At least as early as 1915 intelligent people were writing that burning fuels like petroleum is a criminal waste of a fantastically valuable and limited resource; it's robbing future generations in order to turn our shared treasure into atmospheric poisons. It's nice to see people like yourself thinking about these things!
Look into the difficulties they have had removing sulfur from diesel fuel. Sulfur was added (or not removed) to increase the lubricity of the diesel fuel, to improve freedom of motion in injection pumps. That being missing is a hardship for older diesel engines whose pumps depend on it. The fuel is basically the same in other respects, but for this lubricity factor. This is easily restored with as little as 2% biodiesel.
So here you have these two liquids, mineral diesel and biodiesel - that are both seemingly the same. But one is "slick" and the other not. That property is lubricity, and that is the very thing you need in a lubricating oil.
Water has none.
