Is it hard to become a navy sniper?

Very. You need to get through Underwater Demolitions Training (UDT), which is tough; and THEN you have to apply for and be accepted for SEAL training, which is VERY tough. Then you have to survive. SEALS are the toughest guys I ever knew. THEN you have to be an expert shot.

Want to become a sniper? Be a Marine. If you graduate from infantry training and are an expert shot, you'll have a chance at Marine sniper training.