Story highlights Israel built a 125-mile fence to keep migrants out, but new fears have arisen

ISIS militants are active in north Sinai

Smugglers also try to cross the border, and they're getting more violent

The Gulf of Aqaba (CNN) "Our threats are immediate. It's something like 45 seconds for a threat to start and to finish. There's no time for mistakes, no time to ask questions and to misunderstand something," says Capt. Matan.

"The moment you see a threat from the sea, you've got something like one minute to deal with it and to finish it, and that's it."

Matan, who did not give his full name, is in charge of an Israeli Navy patrol boat, and he's guarding one of the borders his country is most concerned about: the waters where Egypt's Sinai Peninsula meets Israel.

Two different worlds collide in the Sinai.

The southern part of the peninsula is famous for its resorts. Sharm el-Sheikh and Dahab boast world-class scuba diving and luxury hotels along the Red Sea and the Gulf of Aqaba.

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