At the moment, my players are investigating a black naval ship that unknown to them is a trap. It will end up taking them to an island for a trial.
I want the travel of the ship to feel like it takes a really long time. What things could I do on this ship over 1-2 sessions that would make this travel feel long?
While on the seas, narrate storms, sails on the horizon, rations getting padded with fish yet steadily growing leaner. Perhaps the crew growing longer beards, as they only dare shave ashore - tetanus is a thing without safety razors, especially on a moving ship.
Seasonal shifts on the seas are less readily apparent to non-sailors than on land, but sneak in crisp spring breezes, sweltering summer days, rains in the fall and shivering sailors sharing body heat and maybe some snowfall in winter.
If you can show them how far they travel, they will inevitably get the feeling of time as well.
Make landfalls with ever more exotic locales. Perhaps they will have heard of the first stop, as being on the far side of the continent, but the second stop will be very unfamiliar, not to mention the third.
It would probably be useful for the stops to be in cities, to add in languages and merchants who "know a little of their language" at first. But if that creates issue with the story, then have the ship stop to hunt and otherwise gather rations in i.e. rain forests, deserts or polar regions, with possibilities of encountering (dangerous?) unfamiliar wildlife.
Depending on the length of the journey, it's likely that at some point a player character or two will realize they have missed their birthday(s), as they loose all sense of time on the seemingly never ending journey.
Living on poor rations for extended periods of time is likely to cause health issues - cabin fever is a thing, and scurvy is more than a pejorative adjective. Player characters can grow physically or mentally weaker, as the unaccustomed travails of travel slowly overcome them.
