My understanding is that they are inherently unstable for two reasons:
1. Very small keel – that is, they don’t go very deep under water, because ferries tend to pull into shallow areas to disgorge passengers. While that’s OK, the lack of depth means that there is inherent instability in the motion of the ferry boat, because one of the roles of the below-water portion is to keep the rest of the boat stable.
2. Ferries – particularly car ferries – tend to be very wide and not terrible tall. The problem with that is, once again, stability. A flat boat is somewhat more unstable than one that is more vertical. Consider the stability of a raft versus the stability of a rowboat, for example.
And finally, as someone said above, in Asia, ferries are often overfilled with people