The vast majority of it is seawater, which humans can't drink because of its salt content. There's still enough freshwater in most places, but the regions where drinkable water scarcity is a problem are growing larger every year.

Desalination plants do exist, but it's up to 10x more expensive in most places (in California it's only about 2x more expensive because water is already pretty scarce there) and it's also not great for the environment either:

"The problem is that the desalination of water requires a lot of energy. Salt dissolves very easily in water, forming strong chemical bonds, and those bonds are difficult to break. Energy and the technology to desalinate water are both expensive, and this means that desalinating water can be pretty costly...It can cost from just under $1 to well over $2 to produce one cubic meter (264 gallons) of desalted water from the ocean...But switch the source to a river or an aquifer, and the cost of a cubic meter of water can plummet to 10 to 20 cents, and farmers often pay far less."

"There are environmental costs of desalination, as well. Sea life can get sucked into desalination plants, killing small ocean creatures like baby fish and plankton, upsetting the food chain. Also, there's the problem of what to do with the separated salt, which is left over as a very concentrated brine. Pumping this supersalty water back into the ocean can harm local aquatic life. Reducing these impacts is possible, but it adds to the costs.'"

https://scientificamerican.com/article/why-dont-we-get-our-d...