I see lots of this type of system around - it seems an obvious solution but does have limits, some of which are below:

1. The pipe radiates heat and as it is not enclosed in a panel the heat escapes so reducing the efficiency. Also wind will cool the pipe if not enclosed in some form of panel.
   
2. The quantity of water is limited to the amount that will fit in the pipe.
   
3. The surface area of the pipe in contact with the water is small. The bigger the pipe the less contact.
   
4. The surface area of the pipe exposed to the sun is small as large parts of the pipe are in the shade.
   
5. Plastic pipe is not a good conductor of heat.
   
6. At night the heat will be lost due to lack of insulation.