People here have said that rapid change of temperature is really hard on them, so it may be just the coldness of the water. I also read here that heat and cold both increase your body's production of the enzyme that breaks down acetylcholine. Even people without MG notice that they feel weak if the temperature is very hot or very cold. Since people with MG need extra acetylcholine (because we have fewer working receptors), our reaction to heat and cold is more extreme.