Advice

Hello Everyone, not only am I new to these forums but I am fairly new to GPS systems and I am looking to purchase a good system for my boat but don't really have a lot of money to throw at it. Can someone please help me out with some advice on some options I may have.Is there on;y certain gps systems that can run marine maps? how do you tell if it can run marine maps/mode or not? What is the best thing to look for in a gps? I have been looking at places like deal extreme and tiger direct at their systems but I don't know how to tell if it is going to work for what I want to do? Any help would be greatly appreciated. Thank You all in advance for any assistance you may be able to provide me.