So having worked at a whole slew of manufacturers as a tech, the water cooled turbo does absolutely nothing for the turbo on a shutdown. When the oil supply is shut off, and the turbine is still spinning 100,000 plus rpm, oil supply shut off, bearing is taking the beating, even with the synthetic oil. Ford for the powerstoke wants the engine idled 10 minutes after certain parameters are met. In PTS it’s recommended the oil temp be 194 before shutting off engine. I personally have a 19 F350 SRW to tow my 42 foot grand design 5th wheel. While towing oil temp can get up to 220 degrees, it takes the truck almost 15 minutes of idling to drop the oil temp to 194 degrees. With the truck unloaded oil temp runs 206-210 and takes 3-5 minutes to bring oil temp to 194 degrees. I always idle the truck to cool down, bc as a ford tech I have replaced and have seen several replaced turbos due to hot shutdowns. You can always hear the engine on a hot shutdown as well as it’s rough. On a proper shutdown engine shuts down nice and smooth. Moral of story, turbo cooldown is important, don’t matter if it’s water cooled, doesn’t help bearing in turbo.