This is likely due to a mismatch between the resonant frequency of the default cavityCircular antenna and the transmitter frequency. To avoid this problem try using the design function with your antenna object to match the transmitter:
fq = 6e9;
ant = design(cavityCircular,fq);
tx = txsite('Name','Ship 1','Latitude',32.7294, 'Longitude',117.6908, ...
'Antenna',ant,'AntennaHeight',34, ...
'SystemLoss',0, 'TransmitterFrequency',fq, 'TransmitterPower',10)
%Ship2 Recieving Site
rx = rxsite('Name','Ship 2','Latitude',33.0833, 'Longitude',117.9832, ...
'Antenna',ant,'AntennaHeight',34, ...
'SystemLoss',0, 'ReceiverSensitivity',-89)
%Create the propagation model for heavy rainfall rate.
pm = propagationModel('freespace');
%Calculate the range of transmitter using the rain propagation model and a
%path loss of 89 dB. Output is in meters.
r = range(pm,tx, 89)
%Calculate the pathloss at the receiver using the rain propagation model.
pl = pathloss(pm,rx,tx)
%Signal strength
ss = sigstrength(rx,tx,pm)