Yes, lake Idro ist a lake surrounded by mountains at the most southern part of the Alpes. This means that the temperatures in spring will greatly depend on the winter before. It should be a problem to take a bath in the lake after the end of May. The average water temperature is between 20 - 24 °C in the months of July until August... an ideal temperature for taking a swim.
In the spring and autumn the weather is pleasantly refreshing and perfect for a delightful hike, during summer times the temperature will most likely be between 28 - 32 °C outside during the day and between 20 - 24 °C at night. Is there even more to wish for?
Another important information: At lake Idro there are almost no mosquitos or biting flies.