Can you post a pic of the water spots? If clay bar didn't work, I have another suggestion, but here's how to clay bar...
Wash with soap and water, then rinse. Then make up a new fresh bucket of soap and water. This will be your "lubrication" while rubbing the clay bar over your paint finish. Take a wash mit and soak up some water/soap, and squirt it onto the part of the truck you're working on. Then take the clay bar (use small pieces at a time so if you drop it, you can throw away and get a new piece) and rub it back and forth on the paint. You don't have a push too hard, but hard enough so it's getting good contact. This should remove water spots, some tree sap, overspray, etc. If you see discoloration or little dark specks in the clay, then it's working. These are the contaminants or oxidation coming off the paint. Try to use a light colored clay bar (like Meguiars Quik Clay - it's white) so you can see the difference. When it starts to look dirty, roll the clay over a few times and knead it a bit so you have a fresh surface. I use one piece of clay every time I detail and, like I said, if you drop it once, toss it. Otherwise the stuff it picks up off the ground will be etched into your paint.
Whew! Now that I explained that... if the clay bar did not remove your water spots, move up to something more abrasive (but still mild), like Meguiars Deep Crystal Paint Cleaner (#1 in a 3 step product line). This product simply buffs on and off by hand. You don't let it dry like wax, you just buff it on, rub it around a bit, and buff it off. This will remove light swirls, scratches, and anything the clay bar couldn't, especially those water spots.
Let us know if any of this helps.
