Clark had made it to shore along with Ella. Maria was right behind them while Isa was using the current in the river to try and catch up to Reese first. Bard and Grace were just trying to get across themselves. It was good that they recognized that they would be more of a hindrance if they rushed themselves to attempt to help Reese as well.
The living mist was a wind and water type beast. It has some control of water and was able to push against the current slightly. This allowed the ice maiden to use its skills to freeze some of the water being slightly held back. This alone created a float for Reese. If the ice maiden had been stronger, it would have been able to freeze a barrier stopping the river temporarily.
Dungeons are a mystery. but they hold history too