Having used GPS''s by a number of different manufacturer''s, I am a big fan of the Garmins in terms of ease of use and quality of construction. I still use my original 5 year old Garmin in the cockpit, and it has worked without fail through some moderately tough useage.
Today you can get one of Garmin''s more basic marine models and get almost all of the government markers already loaded. I would spend a little more to get a marine model probably in the 70 series, (For $220 I would probably get a Garmin 76 without a mapping function). For use on the Intercoastal, I would probably spend the extra money and get one with a built in WAAS reciever because the 300 foot error circle of standard GPS could and would easily get you aground. The 76 includes a generous 500 programable waypoints and 50 programable routes. Today, almost any GPS have a MOB function and all of the standard navigational output that one could ever need (except that cross-track error seems to be missing in many less expensive units.)The Garmin 76 usually comes complete with a PC cable which allows downloading of updated waypoints.