Some large fraction of the radioactives will STILL end up underwater. A "dirty radioactive bomb" is more a scare word than an effective weapon. To disperse any distance at all, the radioactive material needs to form VERY SMALL particles. Only a tiny fraction of the solid material around the chemical explosive will be fractionated into this particle size range--the rest will be relatively large particles that will rapidly drop out of the air.
Another point is that most car ferry decks are pretty much ENCLOSED, again minimizing dispersal of any airborne material.
No--FAR more effective to set it off at some major intersection in the downtown business district.