A boat GPS unit helps navigate in lakes where you have never been, avoiding hazards, finding and returning to favorite fishing spots. GPS stands for Global Positioning System. These systems give you GPS coordinates by latitude and longitude. They are very accurate, within reason. For the general angler these GPS units will be accurate to about a 5-10 foot radius of the actual position coordinates on earth.
There are several major manufacturers of combination sonar and GPS units that provide excellent marine navigation. For most freshwater fishing boat applications a combination boat GPS/depth finder works well. For larger salt water vessels, many owners invest in separate units to ensure you have maximum navigation resources. Units manufactured recently are mostly digital and use a lot less power than models of several years ago - which can be important for maintaining your batteries power all day. The newer boat GPS units feature internal antennae to capture satellite signals. For larger salt water vessels an external antennae is often helpful.
Hand held GPS units will work in many cases. They will certainly give you GPS coordinates and you are able to mark your way points where you want to come back to fish in the future; or you can mark a particular hazard that you want to avoid. The screens are smaller, but on familiar bodies of water they can be very useful. As for navigation aides on larger bodies of water a larger GPS/Sonar combination unit is helpful.
Marine Navigation is perhaps the best part of these combination units. The two pictures below are of a map of Rainy Lake, Ontario. You will see wear and tear as well as markings where fish have been found. The first picture shows a rocky outcropping with rocks at or near the surface of the lake, coming up from a bottom of nearly 100 feet. It is one thing to see this on the map, but quite another to have a similar map on your Boat GPS unit and use it to navigate on the water to find this specific spot. This is fantastic way to explore new places to fish, look for unique structure on a paper map, and then drive to that spot safely using your marine navigation software, and if successful, mark a way point so that you are able to return.
When you think of a portable GPS unit you often think about a hand held unit like the Garmin shown above. With a little thought you can take a typical Combo Unit and turn it into a portable GPS/Depth Finder. You will need to think about what you will need to do this but it can be done. Using readily available parts as shown in the following picture and with the following items to make the project complete. If your unit requires an external antennae and your current antennae is permanently mounted on your boat, you would need to buy an additional external antennae.