Lack of a ground connection can cause crazy SWR readings.
Lack of a ground connection will make it impossible to transmit.
Lack of a ground connection doesn't affect your ability to receive.
Faulty coax cable between the radio and the tuner can cause high SWR readings.
You need to check that there is a low impedance path from the antenna tuner ground to the sintered bronze plates on the rudder skeg.
The best ground connection between all components is flat copper strip, but wire will work as long as it is multistranded and low impedance.(we have wire from the radio to the tuner to the ground plate connection in the lazarette, and it works fine).