The following chart lists the international frequency assignments for the
VHF marine radio service. All frequencies are in MHz.
All channels indicated by an asterisk are simplex only.
Generally, in the United States, all channels, except for marine
operator channels, are simplex
operation on the designated transmit frequency (there are exceptions in some remote areas).
For example, channel 78 US, or 78A (alpha) transmits and receives on the same frequency of 156.925 MHz.
Most newer radios have both the International and U.S. channels so pay attention to your radio settings.