I don't know about that. Boats are probably softer targets than airplanes. They can be hit anytime while at sea or in port and you can thousands of people on those large cruise ships.
But one I thing I do notice since 9/11 is that your seeing less americans traveling abroad on vacations. I live in Seattle and the alaskan cruise business is absolutely redhot right now. And since 9/11, Hawaii has been setting record numbers every year for tourism.
I've heard from my aunt whose a travel agent that many US travelers are refusing to visit france anymore, and after the muslim burning half the country down last year, US bookings to France have completely fallen off the cliff. Italy is the destination of choice now for US visitors who want to spend time in Europe.