Is ordering tap water in a restaurant rude?

Well surely you’d just ask for a small portion of a main? A side and a starter or a child’s meal isn’t much at all.

And if you’re on a diet or something surely you work out which are cheat days.

The restaurant isn’t a charity.

Tap water is fine to order.

Sometimes I just want to drink water. Most of the time actually. it has nothing to do with money.

too many drinks you risk getting artificial sweeteners and ending up with a migraine.

if the restaurant had a more appealing alternative I would order it, but they rarely do

No, it's fine. A good restaurant will bring a jug of tap water without you having to ask for it.
But her fiddling about with a kid's menu is off-putting and cheapskate (and a decent restaurant wouldn't even have a children's menu!).

I don't like soft drinks so if I don't want to drink alcohol I just drink table water ( unless its a cold day then I might get a tea).

Even if I do order alcohol, I still order tap water to drink with it.

If she has a small appetite I can see why she does not want to order food she will not eat. A starter and side is probably the same cost as a main anyway.

This is actually quite a weird thing for you to be judgemental about and it makes me wonder if you just don't like her that much.

We always order tap water alongside other drinks. It's quite normal.

Why is it better etiquette to order food and then leave it because you can't eat it?

Well is she ordering water because she wants it or because she's tight and resents paying to go out? Same for food - is it appetite or frugality? It's fine to order what you want rather than feeling you have to spend X amount even if it's wasted.

No I do not think it is. My brother runs a bar and I used to help reasonably often when required. Many people will drink simply tap water. In most parts of the country it is a requirement that bars should offer tap water (I believe that this came from the days of ecstasy. That is assuming of course that there are other people in the group eating and drinking and you have not just popped in and said give me a free tap water and sat their spending no money at all!

Behind the bar we would just assume you were the dedicated driver. We and some other bars used to offer DDs free coke/soft drinks.

I'd say it depends on the situation. If the restaurant is packed, then yes, taking up a table for a salad and a water is a bit cheeky... but given what a salad actually is, they probably make a good margin on it too. Otherwise, money is money. Any cheap table is better than no tables.

Some might argue that if the restaurant is that busy, that they can afford it... it works out in the wash and probably gets factored in.

I don't think so, our tap water tastes nice (in my part of the country). I rarely drink fizzy juice. I'll usually order wine or a cocktail or whatever plus a tap water, but if I'm not drinking I do tend just to drink water.

Not ordering a main, or at least starter + dessert is off though. It's not exactly rude, just... inappropriate? Poor etiquette?