Milk is preferable, with a splash of water. I use oat milk, but water is fine in a pinch. I add such a small amount of salt that it may as well be a placebo.

Here are my hard-earned porridge cooking tips, not that you asked:

Start soaking the oats the night before in the pot you'll cook them in (in the fridge if you're using dairy). This softens the oats and reduces cooking time by a lot.

First thing in the morning, cook them on the lowest heat for about 20 minutes while you do your routine. This way you don't need to stir them constantly, they're extraordinarily creamy, and you won't burn the oats to your pan.

Then add a touch of cream (or milk) to your bowl to cool the oats down some, and add some maple syrup if you're into decadence. Ultra low effort, maximum effect.

And if you want to get real weird with it, add some pomegranate seeds. I don't generally bother, because pomegranates are a bit of work, but it's awesome.