Franz,
From the FLETCHER class drawings, it was stated that for FOUR 50-cal MGs, a 45-Gal tank was required. That tells me that each MG required 10-12 Gallons of water (I wondered if they used a coolant additive as well?) for cooling. I doubt that they ever used "portable" water tanks and the pump would need to be attached to something and have electrical power for it to operate. Almost every gun on almost every USN ship in WWII, required some sort of interfaces to the ship. Even 20-mm guns needed electrical power for the Mk 14 gunsight. Installing 50-cal MGs wasn't a simple install.
There were "mobile" 50-cal MG mounts, that probably had everything mounted on the cart needed to operate the gun. It has been awhile since I read about them in textual records, but I think that they had prepared positions for installing these. They were intended for temporary locations where removing them would be desirable for some onboard operations. If you look in this image of USS PENNSYLVANIA ...
http://navsource.org/archives/01/038/013801e.jpg ... on her fore deck are two of these guns. Why they still had 50-cal MGs onboard her in 1945, I don't know.
Also, in the design work for where and how to add 20-mm guns (along with the 40-mm guns) on these old battleships, they NEVER planned to install 20-mm guns in the birdbaths. The top weight concerns of the added weight for the 20-mm guns and ammo, was a prohibitive issue. Actually it was decided early on that the main mast tripods would have to go. Plus getting ammo atop those tripods from a clipping room would have been a chore.