Q: very loud squealing sound about 15 minutes after driving

Well, if you’ve removed the belt and still get the noise, that eliminates most of the normal causes like alternators and idler pulleys. I doubt it has anything to do with the water in the cooling system although you absolutely should change that to the proper coolant. Even if you live where the engine will not freeze, the antifreeze has certain corrosion inhibitors that you need in the system and a proper coolant mix has a higher boiling point than water. Anyway, back to the squeal. Sometimes, a vacuum leak can cause a whistling noise that sounds a lot like a slipping belt. The noise would go away when you step on the gas because the vacuum would be lower. If you can get a piece of 1/4 inch plastic hose about 3 feet long and put one end in your ear, it will make a highly directional listening device. You can point it around in the engine compartment and zero in on the sound’s location pretty easily. If that doesn’t work for you, contact YourMechanic. They can send a technician to your home or office to check out your squealing noise and let you know what it will take to fix it.