How do you sneak onto a ferry?

I'm tempted to trash this in the "illegal acts" category, but there is at least the possibility that under some circumstances it might not actually be illegal (doing so to avoid paying the fare is, though).
You sneak onto a ferry the same way you sneak anywhere... sneakily. It's hard to answer more specifically without knowing what ferry and what the circumstances are.
well first you have to look like a ninja. then next make sure you jump on the end of the dock right when they are about to leave so you go unnoticed. next if there are any guards watching... shoot them. oh yes i forgot to mention that you have to bring two guns with you. the first one must be a silencer so no one hears the guards get shot. the second is of your choice ;)