I think in any normal English "on" and not "in" would be used here.
You're right. Another possible translation is "There are many tourists on the big ships."
Yes, and also in Hungarian it sounds better: a nagy hajókon.
Sok túrista van a nagy hajókon.
A nagy hajókon sok túrista van.
Why not hajókon?
I had there are lots of tourists on the big boat. This should also be accepted right?
Boat is a smaller vessel, more like "csónak".
I did that too, but see now that 'hajókban' is plural, so boats or ships, not boat.
Interesting "in". Means below decks. More likely to use "on", which can mean on deck or inside, non specific.