United States Radio Watchkeeping Regulations 47CFR80
FCC 47 CFR 80.310 ... U.S. recreational vessels NOT required to carry radios:
"Vessels not required to carry a marine radio (e.g. recreational vessels less than 20m length), but which voluntarily carry a radio, must maintain a watch on channel 16 (156.800 MHz) whenever the radio is operating and not being used to communicate. Such vessels may alternatively maintain a watch on VHF channel 9 (156.450 MHz), the boater calling channel. Note however that urgent marine information broadcasts, such as storm warnings, are announced on channel 9 only in First CG District waters (northern New Jersey, New York and New England)".
Translation: If the radio is ON, and you are not 'communicating' then you need to maintain a 'radio watch' on Ch 16/(alt 09) .... 16 (+9?) in First CG district. If the radio is OFF, then you dont need to maintain a radio watch when you are a operating a "NOT required" vessel . (This 16/09 'watch" requirement is accomplished by "tri" (16/09/13) or "dup" (16/09) scan function on Icom radios.)
FCC 47 CFR 80.148 ... U.S. vessels required to carry a marine radio:
"U.S. vessels required to carry a VHF marine radio, such as commercial fishing vessels, must maintain a watch on channel 16 (156.800 MHZ) while underway whenever the radio is not being used for exchanging communications".