You might be able to 'do' Islands in half a day (even then I'm sceptical), but you can't appreciate it in half a day. The list of little easter eggs, gags, tiny details and hidden things is as long as my arm in Port of Entry alone, before you even get out into the park, and then this only continues with the Sneetch Beach, Mythos Troll, ect ect. Add to that all the experiences such as Poseidon's Fury, the water rides (any park becomes a half-day if you cut half the rides out...), the obligatory Croissant Moon and Butterbeer stops, and you've got yourself 9 'till 9.