It largely depends on your metabolism and your fat insulation.
About 9°C is the bare minimum I can go without getting feet too numb by cold water. What's very important is to get heat from whole body and head by using a dry suit with hood: as long as I'm getting hot, no problems going barefoot in cold water.
BTW in Silvaplana during summertime, I'm always barefoot with either a shorty or a short arm steamer. I just hate booties.