I've got 2 boats with Lowrance GPS units on them and I've gone in the the daytime and laid out some good tracks through some tough to navigate waters.

I've found that about the max speed that I can make and still have the GPS update fast enough to allow me to follow the track accurately is about 4 mph. If I go faster than 5 mph I'll overshoot the turns and end up having to overcompensate to get back on the track. This will tend to wack me into things or onto things like sandbars! At 4 MPH I can follow the track exactly in complete new moon fogged out conditions. They are really quite the tool.

I dont mean to sound like Im trying to top something, but we have an area the same situation as you and we go full speed (30ish in an outboard) to fit in a small channel that runs through the middle of a bayou/creek and have no problem? Its just one of the basic yellow garmins with no map, just tracks and waypoints and it updates all the way to 90 degree turns pretty quickly. Are you in a covered area where signal cant reach easily or is your area so precise/obstructed to require you to putt-putt anyway?

Get a delorme pn 30 camo. I asked for reviews on it and didnt get any but still decided to buy it. It makes my garmin 60csx look like a tinker toy. With the downloadable maps i have full arial USGS photos and water depth that make navigation 10 x easier. They load fast too. We tried following tracks in the middle of the night and its on point a lot faster than my garmin. Plus it comes with topo free.