In most cases, poor GPS accuracy is a result of poor antenna positioning or terrain. The signals from GPS satellites are relatively weak, so just about any object will obscure the signal. If your antenna is flush mounted to your rear casting deck, for example, then you, your passenger, your console, your outboard, or your rear pedestal seat could block satellite reception. The antenna that comes with the Humminbird combos is a good antenna, but only if it's mounted properly. Mount the antenna as close to the center of the boat as possible and above fixed objects that might block reception. Put it on a stem if necessary, but don't go so high it'll be a hindrance to casting.
The effective reception area for a GPS antenna is from 10 degrees above the horizon to directly overhead for a full 360 degrees around the antenna. The more satellites visible to the antenna, the more accurate your position can be calculated. Every satellite continuously sends out out a signal that basically includes a time stamp and the satellite's position at that time. The GPS unit uses the time stamp to calculate how long it took the signal to travel from the satellite to figure your altitude, and uses the position information from the satellite to triangulate your latitude and longitude, correcting for altitude. For a GPS to effectively generate a 3D position, the antenna needs to lock on to at least four satellites. A GPS can 'guess' your 2D position fairly accurately using only three satellites, by assuming your elevation above sea level, but you know what they say about assumptions...
Cloud cover, atmospheric distortion, and even the magnetic field of the earth all affect how long it takes the signal to travel from the satellite to your GPS unit, adversely affecting accuracy. To offset interference and distortion, WAAS was developed. Simply put, there are GPS receivers scattered across North America that know their exact 3D position on the globe. They calculate their position based on the signals they receive from visible satellites and compare the calculated result to their known position. The difference is used to calculate positional error and generate corrections. The corrections are relayed to satellites and then broadcast back to earth as WAAS signals. By applying the corrections to the GPS signals, current GPS+WAAS receivers are capable of accuracies down to +/-1 meter and can easily achieve those numbers under the right conditions. Unfortunately, most rivers run though valleys that are surrounded by tree-covered hills. You can lose a lot of visible sky to that kind of terrain, so even with a properly mounted antenna accuracy may suffer.
Play it safe. *Never* rely entirely on GPS to navigate your boat. Expect your average 3D positional accuracy to be around +/-3 meters, even on fairly open water. When you mark a waypoint on your GPS, also use landmarks to visually triangulate your position. Keep a notebook handy. Jot down the waypoint number, draw a triangle around it, and on each point of the triangle, jot down landmarks. I've fished with old salts that are as accurate as a GPS unit. They can effortlessly take you to the same brush piles, out in the middle of nowhere, day after day. That's a skill everyone should develop.
