For your first GPS stick to an inexpensive model from a major brand like Garmin, Lowrance, and Magellan if possible.
Basic options that people consider are:
Color vs Black and White. - Color is good if you add detailed mapping software
Software Compatability - Garmin is known to work with 3 party software. Especially open source (free) software. Thats good when you want to plot a course on Google Earth and transfer it to your GPS. Lowrace, especially the iFinder series, uses proprietary mapping software that is good but costs more money.
Support for 3rd Party Mapping Software - Good models will allow you to buy a chip that contains detailed maps of the area you intend to use the unit. The primary brand is Navtronics. The chips are usually specific to the model or brand of GPS and are expensive so make sure you have a GPS that you like before you make the investment.
One final note. Its been my experience that automotive NAV unit/GPS devices do not make good marine units. Someone else might have more experience trying than me and have a different opinion. The iPhone and Google Earth are a great way to look at your current position if you have one handy but I would not recommend relying on it for a full featured GPS. I may explore the app store a little more to see if there is something else to use on the iPhone........