The pontoon, stranded for hours, had only two life jackets and no marine radio, Coast Guard Petty Officer 3rd Class Christopher Yaw told The Associated Press from the agency's Ninth District office in Cleveland.
The girl's maturity and the Coast Guard members who used "outside-of-the-box thinking" to shuttle the passengers to safety three at a time with a craft designed for winter use after a salvage boat couldn't reach the pontoon in shallow water helped bring the dangerous situation to a safe conclusion, Yaw said.
"To use the inflatable skiff, that was some excellent thinking on their part," he said. "The 13-year-old stayed calm and collected through the situation."
The Coast Guard declined to give the names, ages or hometowns of the people on the pontoon.
According to a Coast Guard statement, the crowded 16-foot boat went out on the river about 5 p.m. Friday. At some point, it became grounded in the river's Lloyd's Bayou, near Grand Haven and about five miles from Lake Michigan.
At about 1:15 a.m. Saturday, the private salvage boat Tow Boat US radioed the Coast Guard, saying it had gotten a call for help from the pontoon boat but wasn't able to reach the caller again, the Coast Guard said.
A Coast Guard "watchstander" in Milwaukee called the girl's cellphone and determined the pontoon's location, Yaw said.
The tow boat headed to the scene but found the pontoon grounded in water too low for a direct rescue attempt, Yaw said. Meanwhile, a Coast Guard vehicle on land drove near the site, and the crew decided to launch the inflatable skiff from shore, he said.
Coast Guard personnel combined skillful communications with "training and rescue techniques that are uncommon to summer operations to successfully bring everyone aboard the pontoon to safety," Petty Officer 2nd Class Greg Mattson in Grand Haven said in a statement.
It took about an hour to shuttle the passengers to the Tow Boat US, which then took them to the Holiday Inn Grand Haven, where emergency medical personnel evaluated them, Yaw said. He said he didn't have details on the passengers' conditions but believed there were no serious problems.
Three of the people who lacked their medicine suffered from asthma. The fourth needed medicine related to a recovery from a car accident.
Yaw said the pontoon boat emergency shows the value of having a marine radio, even on inland bodies of water, and of making sure that personal flotation gear is used, not just stored somewhere on the boat.
"It's generally recommended that you not only have enough life jackets on board but that you wear them," he said.