Sure, 70% of the earths surface is water. 97.5% being salt water and 2.5% being fresh water. You can't use sea water to irrigate agriculture. You can't drink sea water from the faucet. Additional money/ cost and work would be need to make the water drinkable. Please consider that many states like California, Nevada, Arizona, Texas, water is a valuable resource. And yes, we are running out of cheap drinkable water. That's why most of the west coast gets most of their water all the way from Colorado.
desalination exists. the key word in is cheap. It's an economic and energy problem, not a water resource problem. They get water from Colorado because its cheaper than desalination. Once it's not, they will get water from the ocean.
also, all water requires money / energy to process before we drink it or use it on crops. It's just a matter of much money / energy you use depending on the source.
The water isn't destroyed. It goes right back into the ecosystem. The planet surface is 70% water. We aren't running out of water.
> how many people could we feed? The US could probably feed the entire world with meat, with unlimited energy / $$. People going hungry is not a food production problem, it's an economic and logistic problem. If we use plant based food, we could probably feed 3X the world problem (or use 1/3rd the land). World hunger isn't caused by lack of productive land.
Your other point on meat requires more land use and energy than plant food is correct.