Regarding whether a listening watch on channel 16 is compulsory when at sea, the ITU say...

All ships should, where practicable, maintain watch on channel 16...

They say "should", not "must". Therefore it is not compulsory. Changed the Wiki page to say advised rather than required, as it is not required in all jurisdictions, even if it is in US waters by the FCC. It is not compulsory in UK waters, for example.