Supposed to be waterproof!!!

Signed up for a contract at Vodacom Centurion on a Sony Xperia Z1 compact which is supposed to be waterproof. Had the phone for say 3 months and took a picture under water (flaps and all shut as requested by manufacturer). Phone died. Took it in for repairs and got the phone back with a replaced main board. After another month or so took another picture with it under water and the phone died again. Took it back to Vodacom Centurion for repairs. After about 3 weeks they informed me that the phone is ready for collection. When I went to pick it up they said it was written of for water damage. So how can Sony or Vodacom advertise a waterproof device and write it of for water damage when I clearly double checked that the flaps are shut.