For any GPS, you will have to buy the maps that download into your GPS. Yes, all mappable gps units sold today have a basic map of the world. The details on this basic map varies from one country to another. As you can imagine, N.America's basic map has a bit more detail however not near enough to satisfy your needs.
For looking at these maps, you need a gps that can show them on their screen. Now a days, almost all gps units can show detailled maps. The best bang for the buck is still the Garmin Legend for about 100 to 150 dollars. If you want a colour screen, it will cost you a good 50 dollars more.
Now...for the Garmin, I'm not sure at what speed it actually starts to show the speed you are moving because there was a while where anything under 2 mph was not shown or was not reliable. I have an old Lowrance and it shows speeds at under 1 km/h. Yes....showing speed is very important for fishermen because speed is so important for trolling.