Electromagnetic (EM) waves are the backbone of most modern communication systems, but seawater is a tough medium for them. Because seawater is highly conductive, EM waves lose energy quickly, and penetration depth depends strongly on frequency, salinity and temperature. At higher frequencies, such as VHF and UHF, signals are attenuated within just a few metres; even in the kHz range, reaching beyond tens or hundreds of metres is difficult and usually requires very large antennas and high-power amplifiers.

The survey explains how skin depth and attenuation increase with frequency, and why only very low frequencies (ELF, SLF, ULF, VLF) can reach larger distances—at the cost of very low data rates and significant system complexity. Nonetheless, EM-based underwater links can still be useful for specific short-range tasks, such as connecting submerged devices without mechanical connectors.

In the UnderSec context, this means EM underwater communication is not a universal solution but a specialised tool that can support local links around critical subsea assets, complementing other technologies in hybrid architectures for underwater security.