There are lots of references for building solar water heaters, but one of the best articles we have come across describing the different types and how they are constructed (and what climates they are useful in) is the Rob Harlan / Michael Hackleman interview (free download) at http://www.survivalandbeyond.net/wp-content/uploads/2012/07/Solar-Water-Heating.pdf, from the September/October 2000 Backwoods Home Magazine issue.
They discuss the 7 major types of solar water heaters, which are the biggest bang for the buck when it comes to using the power of the sun in a practical manner.
1. Integral collector/storage
7. Active closed-loop