The application asks the operating system to receive as precise location information as the device can provide. If the GPS accuracy isn't as desired, I can provide some additional hints.
The accuracy of the application depends on the device used and on the location & surroundings of the device. It is recommended to keep the device somewhere where it can receive the GPS signal without any interference. On most boats, it is OK to leave the phone on the navigation table or close to the bed. Please ensure that it is not close to some bigger metal object that might interfere with the signal. It is usually good to do some testing to see which placement works best.
Based on my testing of the application on a bunch of different devices it is reasonable to expect the device to have GPS accuracy better than 10m(displayed on the main screen of the application). Although that sounds too big of an error, it in practice means that the phone reliably detects relative changes in the boat's position of 2-3 meters. Of course, this is just my experience with a subset of devices & operating system versions, and behavior differs based on the external factors and exact phone used.