As water started fill up in the coffin he was tricked into, Stefan banged on the coffin as hard as he could. He knew it was useless, but he had to try. Thinking maybe a vampire could hear him through the rushing water. He knew he could survive because he was already dead, but he had to ask himself before he went under: Will his brother know that that it's not the real him?
Stefan hoped Damon knew him well enough to know who he was before it was too late. He didn't know when or how Damon will find him, but he knew that some way and some how Damon will find him. Because whatever crap they've gone through because of Katherine, because of Klaus, and whatever else was dredged up along the way, they were still brothers. They still loved each other.
Stefan knew that brothers took care of each other. He knew that there was a way for them to be together one day despite being torn apart by a girl again. One day, he will see his brother again.
Please find me, Damon. It was his last conscious thought before he slipped into a very deep sleep.