When I was a kid, General Mills has the slogan for Cheerios, 'Tasty Os of Toasted Oats'. Cheerios do not, in fact, taste of anything. But oats do taste of something. Oatiness is not, apparently, one of those tastes recognised by wine-tasters or coffee roasters, but it is distinctive nonetheless. That is why I like to have the porridge sometimes with water; if I have four times as much liquid as oats, it is going to taste of that liquid more than the oats.
The recipes I have found use lots of other liquids: milk, cream, half and half, sweetened condensed milk, coconut milk, soy milk, or orange juice. I can see the case for adding milk; a friend tells me that the oats need the proteins. But I have to draw the line on these additions. I could take the confetti from the paper shredder's bin and cook it up with coconut milk and add maybe cinnamon, nutmeg, and date sugar, and it would taste okay. I want to taste the oats.
Despite my post about the purism of Scottish porridge, Scots do, of course, have milk with the porridge, but from what I hear from Scottish recipes or Marian McNeill at Joy of Baking, they add it cold, at the end, and then enjoy the contrasting temperatures and textures as the milk cools the hot porridge. There must be two main tendencies in cooking, those who like to mix and those who take pleasure in keeping separate and contrasting.
Perhaps I am not going far enough in my authenticity in using tap water. At the Alt-na-Craig Guest House, in Ardrishaig, as I recall, they had lovely amber, peaty water, and if they used that in the porridge, it must contribute to the taste.