You're right but, you also can't obstruct a navigable waterway by motoring 3 abreast taking up the entire channel.

If I was coming from astern and wanted to pass I'd poke up with my bow behind but port of the the boat with the white inflatable and give 2 one second blasts from my 'Giant air horn',

At that point I'd just hold my position and watch all the boats to see what they do while listening to the VHF on scan. Don't think I'd even try the VHF for hailing them.

If I get no response I'd contact them through the loud hailer and instruct them to go to their radio and contact me on 16. You know,,, like the C.G. does.