I have an ancient Garmin hand held gps which I've taken on the boat sometimes. It reads to a tenth of a mph.
Typical wash free canal cruising speed is about 3.1 to 3.3 mph. When crawling at tickover speed past lines of moored boats, which as we all know gets really tedious, our speed is still about 2.3 mph so the difference is really not all that great.
On the Thames the current can be over 2 mph when the level is up a bit and with the deep water you can often see 6 mph downstream but less than 2 mph upstream. That makes a huge difference to your travelling time.