Digi always recommends testing the modules in the actual environment to determine the range of the final product. Because every environment is different, the Digi specified communication range is not always applicable though we strive to specify the ranges conservatively.
Digi's experience has shown that path loss over water is greater than that over land. This is due to a difference in permittivity (conduction) and smoothness (water surfaces are unstable). We have customers reporting 4.7 miles across large lakes and XTend radios are frequently used on oceanic buoys. One thing to keep in mind is that the higher the antennas are elevated, the less the water will affect your wireless communication link.
Please refer to this article for tips and guidelines for improving your range: Maximizing Range
Note that Digi RF products are not rated for underwater use. Water absorbs high frequency radio waves, the maximum distance with any Digi RF product underwater would be a few feet at maximum. Traditionally, ELF (Extremely Low Frequency) is used for submarine communication, usually between 3-300 Hz. Digi does not offer a product that would meet this requirement.
Last updated: Aug 23, 2017