I certainly would not put my wedding ceremony at the mercy of the ferries. The seas in the channel can get pretty rough and the ferries can stop running until it settles down. Plus it's not at all uncommon to see someone who has had too much to drink tossing their cookies onboard. Not the scenery I want for such an event, nor would I want it to be a member of my party.
The ferry ride can be absolutely beautiful and uneventful as well. What is your tolerance for risk?