Ouch, the jewels :P
I feel the mountain water was colder than 18 degrees Celsius which is a sea temperature I have swam in without the same pain. But I have no clue how cold the mountain spring was as having your feet in and the rest of your body out can probably create a big apparent difference. Also when you are swimming, you can warm yourself with movement while there is no real physical strain when you are just keeping your feet in the water.
I think I have experienced 15 degrees in the Baltic see in Estonia once, but I honestly don't remember the sensation that well and I think I was lucky to fall upon some weird warmer stream or something, so I have no clue what the actual temperature was and I couldn't use it as a comparison.
Well if you were tingling afterward it was cold! Lots of fun.