"Ahem ! Sir (or madman). The Master presents his compliments, and wishes to inform you that the vessel is presently on fire. He begs the pleasure of your company at your earliest convenience in the liferaft on the starboard side, where tea and muffins will be served. "
Had a neat intercom system on our ships but prob bit expensive not seen it in b&q yet
Dial up any cabin and loudspeak - the bloke in bed can shout a reply or pick up a handset to talk. Easy numbers for captain, bridge, watch officers, engineers etc
Handed a proper emergency once and it was brilliant - in fact life saving
There are always lots of sudden noises on a long passage and crew manage to find a way to sleep through almost anything. So I'd suggest something that would alert them, subconsciously, as it were. A voice message will be useful. So, for hetero male crew, a female voiceover message like "hello big boy, are you ready for me" might do the trick speedily, or possibly a male voice saying the same might be even more effective. You would of course need to programme the message according to gender and sexual orientation, natch. An alternative might be a message from that bloke on Dad's Army, saying something uplifting like "we're doomed, we're doomed!"
Idea number two would be to line the berths with a modified electrical fence system. Just one throw of the switch from central control and everyone should surface fairly rapidly. Those who don't sleep on their backs may not appreciate this option however.
Hope this helps and good luck with the project ....
And if the string wes attached to the person at the helm so if he goes over the side the bells will sound (or pull the approciate piece of the crews anatomy)
Talk of Nauticalia reminds me of their ship's bell. Insufficient I know with tcm's mega 2boat, but a visit to the Whitechapel Bell Foundry should produce something adequate. An A sharp 11 1/2 incher would be adequate without being ostentatious. And it doesn't rely on electrics or pneumatics, just a girt big hammer,
Tacking back onto topic, what about using 4 of these plus one extra push button. Make each individual chime learn its own button with one chime and they all learn the extra button using a different chime. You have 4 push buttons on a board in the cockpit labeled with cabins and the extra that rings them all.