There are quite a few metal detecting forums out there that provide a lot of info on any machine you can imagine. Friendly Metal Detecting is the one I spend the most time on. Check them out and you may learn more than you can imagine is out there.
As for the saltwater hunting, the beginner level machines listed above are very limited for that purpose. Saltwater is very mineralized and drive those type of detectors crazy. That is not to say it isn't possible to find things, it is just much more difficult. There are detectors that handle that environment much better but they are more expensive. If you plan on doing a lot of wet sand hunting you might want to save your money and buy a better model.