Pex is not designed for outdoor use. It shouldn't be exposed to sunlight for significant amounts of time. It's simply not designed for it, it will degrade as most plastics do from exposure from sunlight. Look at:
PEX Products - FAQ
If your really looking for a cheaper way than say new cooper tubing might I suggest the grills from the back of broken refrigerators welded or brazed together, plus your water heater is then recycled instead of using a new oil based product.