Jin and Sun are the last two people left in the submarine, other than Frank's body, wherever it ended up. Sun once again begs Jin to leave, but he insists that he can free her. Sun is crying and says that he can't save her. Jin tries again to free her from the metal pinning her in with no luck.
Sun begs Jin to leave again and he says "I won't leave you. I will never leave you again." He tells her than he love her and then they kiss one last time. Sun tells Jin that she loves him as the water rises even more.
We cut out and see the submarine with the hole in the side slowly falling away into the ocean. Back inside the submarine, the water has filled the room. We see Sun and Jin's hands holding each other under the water and then the slowly slip apart. We can see a small portion of Sun's body slumped over lifeless.
This was easily one of the most emotional deaths of the series. After three years of being apart, Sun and Jin are finally reunited only to die with each other a day later. Jin's deciding to stay with Sun is a loving and emotional decision, but on the flip side, it leaves their daughter Ji Yeon without parents. It's rather devastating.