It all depends on how many satellites and in some cases towers you can connect to.

In most civilian applications, store bought GPS receivers are low power. Some of the better models include technology call WAAS. This allows the unit to lock on to land based signals as well as sat. based. Combining the two can be very accurate.

The only difference while using GPS in a marine environment I can see is there are very get overhead obstructions and if on the ocean, very few WAAS towers.

http://gps.faa.gov/Programs/WAAS/waas.htm