Witnesses said the man is apparently homeless and had been wandering around the marina near Daniel's Broiler, at 809 Fairview Place North, shouting and talking to himself.
"He seemed loud and angry ... distressed about something," said witness Christian Garone.
At some point the man walked out onto the dock and went into the water. He swam around for a time, and refused to take a lifeline that was thrown to him.
Some passersby saw him go under and jumped in after him to attempt a rescue, but they were unable to find him.
Others called 911 and crews responded quickly to the scene. A dive team found the man, brought him up, administered CPR and took him to Harborview Medical Center in critical condition.