The Detex ProxiPen system is the best for this application.
They are waterproof. Drop them in a pond or pool, or off a dock.. Get it back the following day, it still works.
The checkpoints are small round disks the size of a quarter. They are black. You can paint them, you can bury them in plaster. Put them behind a glass window, or on the other side of a door or painting. Discreet. Very discreet.