I've seen a few different designs for solar water heaters (on this site and others) and I wanted to share my own. It is quite an efficient design since every square inch of collector surface is in direct thermal contact with the water being heated. You can easily modify the design to any size you like. I made mine 8ft long by 22" wide so that it can fit between the rafters in my attic. Tests showed that system output averaged about 530 Watts, heating 20 litres of water from 24 degrees C (75 degrees F) to 47 degrees C (117 degrees F) in one hour.
Aside: I'm in the middle of re-roofing my house and plan to build in a transparent section of roof in one area. Then I can experiment with different solar collector designs like this one and install and remove them easily from inside my attic instead of having to go out on my roof. It will make the plumbing easier too. The drawback is that if a collector springs a leak, it will leak into my home instead of into my gutter.
For information on this and other projects of mine see my website IWillTry.org