The quality of water used for a marine boiler depends on the type of the boiler and also its capacity. All the modern marine boilers having high steam capacity, along with high temperature and pressure, require the purest form of water whereas a medium size boiler will not require extremely high water purity level.
No matter how pure the boiler water is, it tends to contain some kind of dissolved salts and impurities in it even after boiler water treatment. These dissolved salts separate from the water when the boiler water is heated and stick to the heating surfaces of the boiler resulting into scale formation. The main disadvantage of these deposited scales is that they reduce the heat transfer property of the boiler drastically, which leads to overheating, failure of tubes and loss of efficiency.
While few salts come out of the solution, there are others that remain in the water to produce acids that affect the internal metal surfaces. As the acidity of the water increases because of more dissolving of salts, the operating stresses on the boiler increases which results in a detrimental condition known as "Caustic Cracking". Caustic Cracking also means the cracking of the metal of the boiler which leads to metal failure in the long run.
Apart from this, there are dissolved gases such as carbon dioxide and oxygen, which sticks to the boiler surface, leading to excessive corrosion of the boiler and feed system. There are also suspended impurities in the boiler water which results into foaming after mixing with excess salts. The foam floats at the top of the water and leads to carryover of water with the steam leaving the boiler drum. This phenomenon is known as priming and is considered a prominent reason for many boiler damages.
Image Credits : Lancashire Boiler by Elsie esq. at Creative Commons/Flickr (http://www.flickr.com/photos/elsie/3073841683/sizes/m/in/photostream/)