Depends how much you like your engine/turbo, the oil heats much faster than the water, so the water acts as a safeguard, esp on 4wd where the oil comes from the head having absorbed all the oil(and crap in it)and HEAT on it's way up the engine, now add it next to a exhaust that can easily glow red, and of course being metal transfer heat everywhere inc the sleeve bearings, then do the sums, the water at least trys to keep it at a reasonable temp, and after switch off helps by acting as a heat soak also.
Yes you can run without it if you want, but when you buy a new car and it is fitted, it is there for a reason, and that is down to warranty claims amongst other things, keeping the bearings/oil within it's operating range is better than not.
Just think how hot your exhaust is when you stop, now add a massive lump of heat absorbing material, we'll call it the turbo in this case, with a tiny amount of STATIONARY oil in it, and think of it's job.
Up to personal choice, but if I had the option I would use the water cooling, it's free and beneficial.
tabetha