There are the explicit water models (TIP3P,TIP4P, SPC, SPC/E) + some other solvents, and a number of implicit models which you can get information about by searching the CHARMM documentation (cd ~charmm/doc; grep -i solvation *.doc).
As to which model works best with which system you have to read the primary literature and form your own opinion. There are a couple of relatively recent comparisons of some of the existing models, eg
Feig, M., and Brooks, I., Charles L (2004). Recent advances in the development and application of implicit solvent models in biomolecule simulations. Curr Op Struct Biol 14, 217-224.