Posted on October 1, 2020 by DillardSea Kayak on Lake Yellowstone The drive down to Grant. Well it did stop snowing before we got on the water. Yellowstone Lake remains cold year-round, with an average water temperature of 41°F (5°C). Because of the extremely cold water, swimming is not recommended. Survival time is estimated to be only 20 to 30 minutes in water at this temperature.