I have a Garmin 45 XL it is a older model and only has 8 satillites. Boughten at a pawn shop for $75.When you want to learn how to use it go to the link above or Geocache.com were you will find things planted all over the place more than likly there is one within a mile or two of your home. I would recommend doing some of these before going out on the boat in the fog.
If you do not get one with a map do your trip once before you have to do it in the fog

NOTE: And always carry a back up (COMPASS)