As you are starting to conclude, GPS technology is a little limited in terms of usability. You now know that it will not work underwater and can be extremely limited in other countries, but it also does not function superbly indoors. Technically, it can be used but it may not be very useful to you. When trying to use the satellites to receive a signal, the walls, roofs, and other indoor objects reduce the effect of the signal. But, there is a system out there that was designed for this very purpose.
An IPS, which is an Indoor Positioning System, is a system that is specifically designed to locate people and objects that are inside of a building. They achieve this using radio waves, acoustic signals, and various other sensory information. However, integration between the two separate technologies can be made. In specific, GNSS receivers that have high sensitivity are able to adequately receive satellite signals in most indoor environments.