In general, any vessel equipped with a VHF marine radiotelephone (whether voluntarily or required to) must maintain a watch on channel 16 (156.800 MHz) whenever the radiotelephone is not being used to communicate.
Source: FCC 47 CFR §§ 80.148, 80.310, NTIA Manual 126.96.36.199.c(2)(e), ITU RR 31.17, 33.18, AP13 §25.2
Interesting that it would be required by the FCC, yet I don't find it mentioned in the NAV regs. Boats.com - Rules and Regulations: Navigational Rules
. They cover audible signals (horns, bells, etc.) and stress that even if the signals are automated, they must be able to be produced manually. Makes me wonder if they don't want people depending on a communication device that is subject to operation or mechanical glitches for Navigation. However, if the FCC requires it, I would think it would be mentioned.