My choice for this task is a cheap GPS from Wal-Mart plus a laptop computer. Get a used laptop out of the want ads for 300 bucks. Get the GPS for about 100 bucks. Download free GPS plotting software from www.sping.com
Download free digital charts from a link found at the same site. Hook it all up and you are set, plus the laptop can do 100 other things like download photos, predict tides, send E-mails describing all the fish you caught, and so on. When you leave the boat you take the laptop with you, just like any 21 century fisherman should do!!