Are you confusing GPS with Mapping? I fish several lakes that aren't mapped. I created my own using AutoChart Pro.
GPS is from satellites in a low orbit around the earth. GPS is always available in the USA and to the US military world wide. Trees, buildings and mountains can block weaker signals, but you shouldn't have any interference on the water. During the early GPS years a ground based GPS transponder added to the integrity of the location, however I don't remember what it added and don't know if its still required. Things changed several times since 70's when it came out.