We do not specify a maximum line resistance. However, the Inductive Modem systems are very tolerant of the resistivity of seawater. It is typically only a consideration when the system is being deployed in freshwater lakes, etc. where the salinity level is a lot lower.
We recommended using one of the two oceanographic cables below or those of equivalent specification:
1. Loos & Co. 3x19 Galvanized Oceanographic Cable, Plastic Impregnated
2. WireCo WorldGroup Macwhyte 3x19 Space-Lay Plastic Impregnated and Coated Wire Rope
The most important aspect is the resilience of the cable jacket. Any cuts, tears, or breaks in the insulation will create a shorter path to the seawater ground which will result in the the level of the signal on the lower side of the cable being attenuated.