I just found the Nautilus Lifeline GPS/VHF radio online. Interesting device. Apparently it is rated to a depth of around 425 feet. It operates on Channel 16 which is stated to be a marine emergency channel. How many boats actively monitor that channel?
Are marine radios set up to monitor Channel 16 while using another channel?
Would cruise ships monitor that frequency?
Proper procedure for any boat captain is to always have your marine radio on channel 16 when you are not using it, in order to monitor the emergency channel and be able to offer aid to anybody you're in range of hearing.
Channel 16 is the emergency channel and the hailing channel, so you can hail another using 16 and go to another channel to talk once you make contact. The coast guard always monitors channel 16. (US coast guard, Mexico? Who knows)
Good marine radios also have scanning capabilities of at least 2 channels and good ones even more. You're gonna find most dive ops use a specific channel, and other dive ops know who uses what channel, so the captain should have his marine radio either scanning back and forth between 16 and his 'home' channel incase the dive shop needs to reach him, or left on 16 at all times.
Commercial ships monitor 13 so to hail a cruise ship you'd probably want to use 13 and if no answer try 16.
This is all US procedures, most of this stuff is supposed to be followed internationally, but....
I've been on dive boats where they use cell phones for communications with their dive shop.
I wonder if anyone has ever stood on a dock, looked at the situation, and said, this is not safe?.
Myself and 3 dive buddies simultaneously looked at each other and thumbed the shark dive in Roatan years ago, took our gear off the boat at the dock before they left and wished all 35 of them well.
Thumbed the 2nd dive in Isla Mujeres with Enriques Unique Dives, told him to take us back in, felt we were in danger of losing our lives if we continued due to the conditions. He offered no refund or discount, so I offer having no problem talking about it.