I have an Austrian friend who tells me that the water temperature has been between 20-22C (68-72F) the last two years with rain on race day. They recently had a heat wave in Europe that drove the temperature up but it is now falling and measured 21.8C (71.2F) today and is expected to fall to near 70F this weekend. No further heat waves are predicted but you can probably expect between 70-72F depending on how the weather goes. Whether you NEED a wetsuit is another question. Pretty much you can expect close to 100% use.