Yes, ClockShark will work when your mobile user travels outside of coverage.
The mobile user will be able to clock in and out, and then the punches will be sent to the ClockShark servers when the internet connection returns. The GPS Tracking features will still work even without cell coverage since they rely on GPS satellites and not the cellular network.
If your mobile user clocks out while not having service, the time will then show up in the Sync tab as a "Sync Queue" to be sent over to the website. Whenever they do reach an internet connection again, they can press the "Sync" button to send the time over and make sure they get paid!
Learn more in Employee Not Showing on the Who's Working Now (Map) Page if you cannot see GPS tracking for a particular employee.