Antennas placed in saltwater have a reduced read range compared to air or fresh water. The conductivity of the water dampens the magnetic field around the loop and attenuates the signal from the tag.
Saltwater antennas can be built that will work reasonably well. HDX works better than FDX. The supply voltage on our single and multiple antenna readers can be boosted to 18 to 20 volts to increase performance. The antenna wire for satwater antennas needs to be a bit larger than for fresh water to have lower resistance and overcome the attenuation.
If the antenna is in an estuary or where freshets occur, the performance of the reader with vary with the water conductivity.