Regarding the volume of water, Lake Constance is the second-biggest lake of Europe. Its 48 cubic kilometers of water are a huge reservoir of warmth or coldness. Hot summerdays are less hot in Constance than in other nearby German towns, like Freiburg, Karlsruhe or Stuttgart. In autumn the lake works like a heating. Then the nights are milder than at other places in southern Germany. And in winter, there are less frosty days.
Does this mean, that the climate around Lake Constance is better than elswehere? Unfortunately the answer is no! Expecially in late autumn and winter, there are many foggy days. Especially on winter-days, when it is sunny in the rest of the country, the sun doesn't show up at all in Constance. Thus, the local climate has advantages and disadvantages.
The impact of climate change is felt for many years now, at Lake Constance. The time, when people swim in the lake has become much longer than formerly. Up from about 1990 the winters were unusually warm, there hardly was any snow around Lake Constance. Whereas in the winters 2010/2011, 2011/2012 and 2012/2013, there were really cold periods and snow. Most recently, especially in winter, the temperature changes have become much more extreme than formerly. The average temperature of the lake is higher than formerly, but even in summer you may catch a really cold period. And then, there are much more severe storms now, which sometimes are not predicted by the weather-forecast. So, if you are doing water-sports, you should be more careful, than 20 years ago.
At any rate, the influence of the lake ist really tangible. And sometimes it is a joy for the inhabitants and tourists (usually in summer) and sometimes it's a nuisance (quite often during winter).