If you're looking for a fast ferry... why not one of the Superseacats that used to work out of Dover...
Lovely boats to travel on. Very manouverable- as we found out one time when the skipper decided to change course rather abruptly. The worst waste of a pint I've ever seen
PS. Yes, I do know they aren't catamerans before someone tells me. Nor are they deep vee monohulls as they are also sometimes called.
Stu
EDIT: Does anyone know what angle a vee hull becomes a deep vee? I was told 35, but just found a source to contradict that.