Fin keel or full keel, a boat doesn't have to be fat.
The Valiant of course isn't quite as narrow as Arion. But after almost 40 years it helps prove a full keel isn't necessary for a boat to be "offshore" capable. There are also many others that have traveled far and wide with fin keels.
I can't think of many full keel boats with high efficiency keels - just large ones with a great deal more wetted surface than necessary.