I've read that solar powered water heaters in the home reduce CO2
emissions by as much as 30 percent. Is this true? Could you expand
upon their current cost effectiveness?

Water heating is one of the best applications for solar, because the load is 
mostly separate from the time of use. 

A total solar water heater has no co2 emissions after it is installed.

Many systems have a small backup heater, elecric or gas for the rare
days of little sun.

Cost depends some what on location. and if it was designed into the
buliding from the start.

Some of the most cost effective solar systems are home bulit masde of
junk pallets, drums, mirrors, because you only have your labor
invested.

Building codes and corrosion of disimliar metals need review, Prior to
buy or make decision.

Stuff reducing CO2
- that is becoming a 'wolf word'

In the right climate solar water heating is very effective
- in Israel it is compulsory