That's only semi true. While a water cooled MG can overheat, it takes a loooooooong time compared to air cooled MG's.
A water cooled MG was designed for the operator to literally just hold the trigger down and sweep back and forth.
They aren't used today because they are crazy heavy compared to air cooled guns.
Let me try to explain this once more.
In real life:
If I was facing Zulu warriors doing a frontal attack I would probably chose to use a water cooled weapon so that I can fire for longer.
If I was facing WWII fighting from fortified position I would rather use the HMG-42 with Lafette 42 tripod that included a number of feature like a telescopic sight or periscopic sight allowing wine to fire without placing himself in line of fire.
If I was a more dynamic battlefield I would rather be using the HMG-42 with bipod and drum belt container due to light weight and mobility.
Weapon like the maxim, 0.50 and Duskha where extremely difficult to move during a fight and having sprint as an ability in game is joke from a realistic point of view.
During WWII in most cases the MG34 and MG42 where superior to most counterparts as squad support weapons.