That is sort of a loaded question.
1. Are you wanting to be able to see actual maps?
2. Do you want to just be able to create and see waypoints.
3. Do you want unit that you can lay down in the seat and see or do you want to beable to stick in shirt pocket.
4. How much green do you want to let go of?
5. How accurate does unit need to be?
There ae several websites that will help you chose the GPS unit for your needs.
http://www.4x4books.com/gps.htm
http://wildasia.net/borneo/main/help_choosingGPS.html
http://4wheeldrive.about.com/cs/gps/l/aa111101a.htm
just to give you a few. I have not found one specifically for fishing, but the points for consideration are similar.
Personally I have Garmin Geko 201 that I jsut purchased and like a lot better than the OLD Lowrance GlobalNav12 that I have used for years. I use a paper map to locate my waypoints on as well as a spreadsheet. From the spreadsheet I can upload to GPS from computer or I can download and backup GPS unit to computer.
Hope this helps.