There’s being creative. And then there’s going overboard. What’s going on at Southwest Plaza, a mall in Jefferson County, may be a bit of both. Construction is underway on a nearly 20,000-square-foot aquarium on the ground floor, a soft pretzel’s toss from the Dillard’s. When it opens later this spring, SeaQuest Littleton will let paying customers mingle with 1,200 sea creatures, including snorkeling in a tank of sharks and stingrays.