Was Spinosaurus the only true aquatic non-avian dinosaur?

It's not really a sure thing with any dinosaur. Whether or not the Ibrahaim et al reconstruction you're referring to is accurate or not is still hotly debated (Sereno likes it, Hartman and Witton are cautious about some of the proportions, and Paul is dead against it). So Spinosaurus might not be aquatic after all. read more

There are a handful of dinosaur groups that are semi-aquatic: Spinosaurus was semi-aquatic, as you pointed out, and some of its relatives may have been as well; Hesperornis and its relatives were a very specialized group of ancient birds, which may have spent most of their time in water; similarly, more familiar examples of semi-aquatic dinosaurs include ducks, geese, diving birds, penguins, and plenty more. read more

But even though paleontologists have known about this particular dinosaur for almost a century, its true form has only just been revealed."The only dinosaur that shows [aquatic] adaptations." This is"the first water-adapted non-avian dinosaur on record," said University of Chicago paleontologist Paul Sereno in a press conference yesterday. read more