The main advantage of a pressurized cooling system over a simple tank running at near atmospheric pressure is that the boiling point of the coolant increases as pressure goes up. This significantly reduces the chance of flash boiling of the coolant water in the hot spots of the heads (near the exhaust valves for example). If local boiling occurs you develop a hot spot as the steam pocket formed cannot carry heat away as fast as liquid water can.
If you are running a large tank of water circulated at near atmospheric pressure the boiling point of the liquid is 212 deg F (100 deg C), raise the pressure inside the engine cooling system to 36 psi absolute 22 psi above atmospheric pressure and the boiling point of the water goes up to 261 deg F (127 deg C). A 30 pound pressure cap on the radiator would raise the coolant pressure to about 44 psi absolute, and a boiling point of 273 deg F 134 deg C.http://www.engineeringtoolbox.com/boiling-point-water-d_926.html
This allows you to build your large water tank as a square shape that can operate at low pressure, and only pressurize the radiator and the cooling system of the engine which are designed to run at high pressures. Makes the large water tank cheaper and simpler to build.
The heat transfer from a common radiator to air is much lower than the same radiator fully immersed in cold water. Especially if the water it is immersed in is full of ice, you have a huge difference in temperature which significantly improves heat transfer. It also due to the large thermal inertia of the large volume of water will over heat slower if it does over heat, this might give you a few more seconds at full power and time to shut things down without injuring the engine if something goes wrong and the engine starts to run hot.