I'm planning such a system myself, but using extra 1" black poly water line used for water supply, which will be coiled over some black epdm roofing rubber. Using 1" should expose a larger area to sun per volume of water. I have used the hang up camp shower bags, and they heat up nicely in about an hour in full sun, a bit longer in a breeze. I know black water pipe heats up in the sun, but don't know how fast or how hot yet.
I think the type of plastic pipe used could be important. The type I'll be using is made to handle pressurization (to domestic supply line level , ~150psi) and is fine sitting in the sun. Some types, rigid types made for domestic waste lines, for example, may not stand up to the temperature, pressure, and/or UV exposure involved in a solar water heating application.