1) The position accuracy of GPS depends on the satellites signal. If the weather is cloudy or raining, the satellites signal may become poor.
2) If your boat (with GPS receiver mounted) is stationary, the position coordinate may be drifting. Then the error of GPS accuracy may reach 15 meters or even more.
However, if your boat is moving with speed 0.5m/s or more faster, the position accuracy will become good enough.
3) Please be sure the mounted antenna of GPS receiver is at least 30cm far from any other antenna, such as antennas for boat or fishfinder.
4) In some country or region, for national safety considering, the local government may take some measures to interface the GPS satellites. Thus the GPS coordinate offset may be different at different time.