The blue circle gives an idea of the accuracy of the boat’s position. The boat has 95 pct chance to be inside the circle. It is visible when the map is highly zoomed.
The position accuracy is derived from the hdop (Horizontal Dilution of Precision) parameter calculated by your phone’s GPS chip.
With our cellphones, in open sea, the accuracy is generally around 5 meters.
If you have a phone that picks up the Galileo constellation, you will have an accuracy to the order of the meter. You can view the list of Galileo compatible phones here.
As of Q3 2018, the new cellphones should be equipped with precise GPS, with an accuracy around 30cm…