Hi zaf13 - and welcome to Mayhem.
First things first - what is your experience in building models and what facilities do you have?
You have a few options open to you:
kit build
plan build
completely scratch
your choice will also depend on what budget you have. There are very few commercial kits of passenger vessels about. These can be broken down into various categories:
Liners / pure passenger ships
Ferries
historic or modern
small, inter-island or coastal ferries
etc.
there is a service called the Model Boats Plans Service which has a few decent subjects you could consider including a couple of cross channel ferries.
a few european manufacturers produce kits of suitable subjects, but they are very expensive (nearly £1000 each!).
there have been a few modellers (a few on here spring to mind) who have produced some beautiful models of classic liners, car ferries and other examples - nearly all of which have been scratch built using information that has followed extensive research.
we are all willing to impart advice here - but all advice has to be based on information you can give us about your modelling experience, wishes, abilities etc.
Carl