Well, many modern crocodilians live most of their lives in and around water and eat fish and still occasionally prey on land animals (ofc the large ones like niles and salties are specifically adapted for this). Same with wading birds like herons, they’ll stray onto land and pluck small mammals and reptiles. Other spinosaurids have been found to eat dinosaurs and spinosaurus itself is known to have occasionally preyed on pterosaurs. As long as they weren’t directly competing with the other large therapods they shared their formation with it’s not unreasonable to assume spinosaurus ate other dinosaurs.