GPS system specs and actual reality seen by end users can be two different things.
I'd suggest that you check with your GPS receiver vendor to see what they say.
As an example, many people have a Garmin GPS receiver, here is how they answer the 'how accurate is my GPS' question:
Quote:
GPS accuracy
Today's GPS receivers are extremely accurate, thanks to their parallel multi-channel design. Certain atmospheric factors and other sources of error can affect the accuracy of GPS receivers. GarminŽ GPS receivers are accurate to within 15 meters (49 feet) 95% of the time. Generally, users will see accuracy within 5 to 10 meters (16 to 33 feet) under normal conditions.
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Raymarine says something similar:
Quote:
Understanding GPS Accuracy Statements
What do the quoted position accuracies in my GPS manual actually mean?
GPS position accuracy is normally quoted as a statistical probably of how accurate the position from the GPS will be.
In standard GPS mode (not using differential corrections), the Raymarine Raystar 120 and 125 sensors will give a position that is within 15m (33ft) of the true position 95% of the time.
In satellite differential mode, the Raystar 120 and 125 sensors will give a position that is within 5m (16.5ft) of the true position 95% of the time.
Depending upon the type of fix, 95% of all the position calculations (one occurring every second) will be within an imaginary circle of radius 15m or 5m from the true position. The GPS is still operating within its specification if occasionally position fixes fall outside of this imaginary circle.
As GPS accuracy is dependent upon the positions of the visible satellites, the above specifications are assuming that the HDOP reading from the GPS is 2.0 or lower.
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What brand GPS do you have and how old is it? My guess is that they will state something similar, unless you are using a survey quality GPS receiver, which most boaters do not have on their boat.