During their time apart, the Doctor didn't dream. Dreaming wasn't common for Time Lords in general, but while Jack was away or in his extended coma, there was no accidental glimpse into the human part of the Doctor's psyche.
Now, he dreamed. Twisting storms and violent rainclouds and a ship that was coming apart. Somewhere he heard Martha calling for him, telling him they had to turn back. They had to turn back, there was too much damage in this section of water. The sentence itself didn't make sense, but to his dream-self it explained a lot of things that were wrong.
A spitfire shot above them through the storm, heading deeper in. The Doctor never dreamed of spitfires. He knew them, but not well enough to have such a beautifully detailed image in front of him. In the dream, he saw it as a sign.
We have to find him, the Doctor responded. He's in the storm.
But everyone was here, on the ship. Mickey, Martha, Donna, Rose, Wilf, they were all there. And the Doctor was steering them into danger and---
He blinked. He was awake in Jack's arms. How long had he been asleep? A few short hours, he imagined. How many more hours did Jack need?