Place the potatoes and water in a large saucepan. Bring to a boil. Reduce heat and simmer 1 hour, or until potatoes are very soft.
Mix the evaporated milk, heavy cream, sour cream, butter, onion salt, garlic powder, salt and pepper with the potatoes. Simmer 2 hours, stirring occasionally, until liquid has reduced by 1/
Scoop out and drain 1 to 2 cups of the mixture and thoroughly mash with a potato masher. Return to mixture.