The darkest skies I've observed under were Class 2 on the
Bortle Scale.
http://www.novac.com/lp/def.php
I'm lucky because my regular observing site has class 3 skies, which is excellent considering it's only a 1hr commute. I want to go to the Texas or Nebraska Star parties (if only just once) so I can experience class 1 skies for myself. To get as dark as class 1........you need to be in an extremely remote locale. IE......in the western USA in the middle of absolutely nowhere.
You probably won't get that great of a night sky view off a ship unless they turn off every single light long enough to become dark adapted, but yes..... the night sky would look amazing while at sea!! The humidity would degrade the sky transparency, but it would still be an impressive sight to behold. Even the most minute lighting on board the ship would degrade the views rather significantly. If they cut the lights for an extended period, it would be fun to just lie in a recliner with a pair of binoc's to scan the sky. (Somewhere arid AND at elevation would be much, much better)
This is a fun resource...........
http://cleardarksky.com/csk/
Besides the observing forecast, it also has a feature that includes light pollution and sky darkness by location in the USA and Canada.