Sony Xperia Z3

I took a contract with MTN for the Sony Xperia Z3, not just because I only like the phone but as a bonus it's waterproof and dust proof, a huge bonus in my industry. In January we went on holiday and thought it would be cool to take under water photos, as it shows in the adverts, about a day later my new phone did not want to go out of flight mode, so I booked it in at MTN to send it to Sony for inspection and a week later I received an sms telling me I must pay for a repair but I didn't even know what's wrong with the phone, so I went to MTN to find out what was going on and they told me that Sony found that the phone has water damage,but it is waterproof and it's been advertised all over on television and newspaper,I would really like an explanation from Sony and a free repair or new device, as this is a waterproof and dust proof device