That’s a good question, and if you’ll permit me, I’ll give the answer in two parts. The acidity of seltzer is roughly the acidity of orange juice, so a squirt of lime in your water is just nowhere close. It is also true that most of what we should be drinking is water. I realize that’s not the first choice for people raised on juice, milk, and soda, but it’s certainly the most healthy.

Remember that I mentioned moderation though? A glass or two of OJ, lemonade, tea, coffee, or seltzer a day isn’t going to materially impact your health assuming you’re not diabetic, and practice good oral hygiene. There are also many worse things for your teeth than seltzer, such as potato chips, and poor oral hygiene. If most of what you’re drinking is water, don’t sweat the glass or two of something other than water. Unfortunately too many poeple drink very little unadulterated water, and it does impact their health.

Moderation is the key.

Why are potato chips bad for your teeth? They are mostly oil and salt.

They’re mostly starch, which the enzymes in your mouth convert to sugar.

It's not what they are made of, it's the effect they have. Bits get stuck between your teeth and gums, and this encourages bacteria to grow and establish biofilms, which once established can start causing tooth decay. Long term, they can cause a lot of damage.

They get stuck in between, and in your gums, in a way that (obviously) liquids don’t.

Can you neutralize the harm by drinking through a straw?

Carbonated water typically have a pH around 4, adding lime juice may reduce it slightly further, but not by a huge margin.

Acidity of most beverages on its own is usually not problematic as saliva (buffered at 6.5 pH) rapidly restores normal pH. It does become an issue if the salivary glands are damaged (as in the case of meth users) or stomach acid comes into contact with teeth.(severe acid reflux or bulimia) However these cannot be considered normal.

The number one cause of dental cavities is always sugar.

Unfortunately, not even plain water is uncontroversial.

He's not wrong, matter of quantity: https://en.wikipedia.org/wiki/Water_intoxication

('Notable cases' include recorded death.)

Well obviously any lifeform has water problems. Like any other lifeform, our life depends on remaining within osmotic balance with whatever we drink and eat.

That said, the amount of ways your body adapts to osmotic balance problems is in the dozens, so poisoning your body with water, while possible, is hard to the tune of it requires something like drinking 20 liters of water in less than an hour. I mean, it seems to be possible but I cannot imagine succeeding at that even if I'm trying. That's 2 full buckets (and that's just LD50, as in, if you're >70kg it's more than that).

There is an exception: purified water. You cannot drink mineral-poor* or distillated water safely in even small quantities. Why not ? Because that will locally, suddenly and very strongly disrupt the osmotic balance in what you bring it into contact with. It probably not kill you (not without repeated application), but it may cause serious issues in your mouth, throat and stomach. It should also not be used to make coffee or tea and drunk, that merely lessens the effect it doesn't stop it.

* mineral-poor water is something used in chemical and medical laboratories

It may be hard to imagine, but it has happened: http://articles.latimes.com/2007/jan/14/local/me-water14

I was thinking of things like fluoride, water chlorination, etc. It's also possible to drink water that is "too filtered" and doesn't have enough minerals in it which can apparently cause issues.

Yeah but that's way way past the point where it's very uncomfortable to drink more water. It's not something that can happen to you without warning (or while you're enjoying the process).

Anecdote: My mother got quite sick while drinking a huge amount of water in preparation for a religious 26 hour fast.

Was she comfortable drinking that amount, or was she forcing herself to?