I am still on the original plastic u-joints with the exception of the one I had to modify with the aluminum shock cap. It is still working great. With regard to the u-joints needing water I understand but this is not a race boat. I run it as a scale model so I rarely give it full throttle and I am running back and forth is a small space in the canal behind my house so they are only running "dry" for a few seconds at a time.
However, the metal U-Joints I have coupling the motors to the drives in the hull do get hot. So far no real issue there, they just feel hot when I pull the boat. Today the boat was running awesome but one of the cooling lines for one of the ESC's came off so I was filling the boat with water as I ran. Luckily I was on my real boat running the RC when I noticed it and was able to grab it. I have a bilge pump system I am working on now so if this happens in the future I will know and the water will be pumped out.
As for the U-Joints in the boat that are getting hot, you can see them here.