I can confirm what Gideon's saying about La Rochelle as we attended the show last year. We went for 4 days and found it to be a great place for multihulls of all shapes and sizes. Unlike most shows everything is focussed around the in water
boats and not the normal 5 square miles of chandlers and boat bits. A shuttle boat runs from the main harbour to the show every 20 minutes throughout the day.
La Rochelle is a great place, a little expensive for hotels and food
but not too bad if you move slightly further away from the harbour area. Hotel
availability is a problem so book early through the tourist board who seem to control the room allocations.