One of the things I visited at Boulogne last week was Nausicaa, a "National Sea Experience Centre". With 30 or so tanks that you can view from all sides, below, and even from bubbles inside, it's quite amazing. (especially if you look from above at the people viewing from the bubbles, portholes, etc. below -- it can give the impression that they have added a few aquatic humans to their collection!)
It takes an hour just to walk around and look at all the animals from all sides. If you want to read the educational posters and watch the movies you could easily spend all day.
I had trouble remembering the name at first though, as I was unaware that they pinched it from a character from the Odyssey. According to Wikipedia, it means "burner of ships" in Greek. I wonder whether there is a political program behind that naming ... ?