It's not gone forever. However, it may be in a less useful place.

For example, a well draws water from an aquifer, an underground reservoir; which is refilled by rainwater soaking into the ground. But if water is drawn out of the aquifer faster than it is replenished by the rain, eventually the well will run dry.

Even if that water is still on the planet, it's not available to your well; and so your well has become useless.

The question is a bit like "If I spend all my money, is it truely gone forever or did it just return to the global financial streams?"

Like with the money, water exists in very different states of usefulness. Sea water, for example, is incredibly abundant, but using it requires desalination, which requires enormous amounts of energy.

Ground water is really useful, because it's where you need it and it's usually pretty clean.

Rain clouds mostly pull their water from the sea. Hence using water e.g. in agriculture will not increase the amount of rain by any significant amount.

Ground water replenishment thus doesn't depend on the amount of ground water spent for e.g. lawns. Similar as your wages usually don't depend on how much money you spend on a holliday.

So if you waste ground water, it's mostly just gone, while you wait for rain to refill it. Sadly, in most regions that happens far slower than people are spending their precious water resources on useless nonsense like a green lawn.

You can't destroy it and it doesn't go anywhere. It just gets moved around and used for different things at different times.

Water lawn > Grass uses water to grow more grass > humans mow lawn > grass clippings dry out > water returns to atmosphere

It's a cycle, but it's not in balance.

There is a lot of water on earth. Most of it is salt water which is not usable for crops or consumption etc.

The graphics on this Wikipedia will give you an idea of the distribution: https://en.m.wikipedia.org/wiki/Water_distribution_on_Earth

The water we use for food production, watering lawns, bathing and toilet flushes is pumped from the fresh ground water, which is only about 0.76% of all water on earth.

When we use water, it will eventually, one way or the other, flow into the sea, where it turns into salt water. The evaporation from the sea will create clouds that will rain and seep down to become fresh ground water again.

The problem is that we are basically taking the tiniest bit and turning it into the largest faster than it can be replenished.

Depends. Sometimes the water gets dirty and needs to be treated, sometimes it evaporates and needs to rain, sometimes it could be reused as is

In very rare cases (nuclear fusion) the water is destroyed into its primitive elements

While the water most likely returns to the cycle, in many places replenishing the aquifer can actually take years, even decades. In those places using too much water means the aquifer keeps depleting and causes a bunch of other problems such as salt water intrusion.

Another factor to add to these answers: if the water has been treated (if it's mains water), then a not inconsiderable amount of electricity (and so carbon emissions) will have been used to treat it, and probably quite a lot more electricity will have been used to pump it around the country. So using water is also burning energy

Sometimes it becomes toxic. Sometimes it's relocated to a different watershed. The only time it is removed is when it's split into O2 and H2, but the oxygen tends to stick around, and the hydrogen will soon reunite with oxygen

Water is a bit more complicated than ordinary ressources, watershed are mostly local and it's hard to transport water over large distances. To make-it simple, saving water in Scotland won't bring more water to the Sahara.

All the water we use comes from rain (snow), and it would either go to the ground where it could be pumped, or join stream them rivers and flow downstream. A part of the water you use upstream will evaporate, and therefore won't flow downstream, which is the cause of big geopolitical conflict, especially in dry regions. This water will still evaporate and at a point fall back on the ground as rain, but you don't really control where (and when) the rain falls, moreover, with global warming, a hot atmosphere can store water than a cold one, leading to "less rain".

Another issue is ground pollution. If you keep the ground clean, you can pump water, people have stuff to drink, farmer can water their crop and so on, but if there is any pollution you might have non drink water in the ground or even contaminate the plant you water with it, meaning that water is lost... forever