I just had an awful experience with a firebelly newt that I bought from Petsmart. He looked perfectly fine but he wouldn't go in the water. He wouldn't eat, either. I thought that he was just stressed from being in the new environment and left him alone for as much as I could. Eventually someone suggested that I check him for any missing digits or cuts. I picked him up for the first time sicne I'd gotten him and noticed that his foot was rotting off. There were three other open, infected wounds on his body as well. None of this had been there when I got him. I spent hundreds of dollars trying to save him and make him content and healthy, but he wound up dying.
Everything I've read has told me three things about newts not going in water: a) the water is dirty (which yours probably isn't, if the other newts are any indicator), b) the newt is sick, or c) the newt is a juvenile and in a terrestial stage, or maybe he just really doesn't want to be in water for reasons of personal preference. I would suggest planning on increasing the land area like Mark said, but I would also suggest that you very carefully look over your newt. Is he eating? Is he active? Does he have any cuts on him or missing digits? Before you do anything make sure he's ok. As I learned, these beautiful little guys can go quickly once they're sick.
