Is there a repeater available for the UAD-300 wireless water alarms?

No there is not.

My UAD300 water alarm system is alarming with no visible water being present.

Remember that this system was designed to be very sensitive, so the smallest amount of moisture will trip the alarm. Most problems experienced with this system are battery related. The battery used should measure 12 Volts using a simple volt meter. It is helpful sometimes to clean the ends of a new battery with rubbing [...]