According to officials, the woman fell off a rented sailboat near the Kemah Boardwalk around 10:30am.
Passengers aboard the sailboat made several failed attempts to rescue the woman and then called the Coast Guard.
A rescue helicopter and crew from Air Station Houston were in the area and responded within 10 minutes.
Eric Lipper of Houston was in his own boat when the rescue occurred, and even took a video of the scene.
"I thought the person in the water was not going to make it," he said. "The captain of [the sailboat] was really panicking. He was leaning over... and I was afraid that we were going to have two people in the water."
The Coast Guard helicopter crew lowered a rescue swimmer and successfully hoisted the woman to safety.
She was transported to an area hospital with signs of mild hypothermia, officials said.
"Considering that the air temperature was in the 30s and the water temperature was in the 50s, this situation could have gone bad quickly," Petty Officer 1st Class Joseph Medina said in a release Sunday afternoon. "This rescue was a classic case of being at the right place at the right time, and also of teamwork between Sector command center personnel and Air Station Houston."