Since they are black and lay flat, they are barely perceptible when installed. It is most convenient to install a kit with an automatic controller. You set the pool water temperature that you desire. The controller directs the water to the solar unit only when the pool water needs additional heat. It requires a much simpler collector design to efficiently heat pool water to only 85 degrees than to the 130 degrees required for domestic hot water.
Many kits use flexible sheeting with water passages throughout and they are sold in rolls.
You just roll them out and attach the incoming and return pipes from the pool or spa. Some of the kits even have Velcro straps so they can be rolled up and stored away during the non-swimming seasons. Although they appear to be just a black sheet of tubes, they are specially designed. For example, the web between the tubes on some have an engineered concave shape to redirect the sun's rays to improve efficiency. Others have dimples formed into the tubes to create turbulence of better heat transfer.