Out of curiosity, what's dangerous about Alpine lakes? I understand that rivers can have currents that can pull and hold people underwater, but I can't think of a risk for lakes other than hypothermia.
I can only answer for the lakes near where I live and that I know, for example Lake Iseo o Lake Como.
They might look all calm and placid, but they are really deep with steep shores that are pretty irregular, think of them as a valley between two mountains filled with water with water coming in from one side and going out from the other (that's exactly what they are actually).
The water flows more rapidly beneath the surface and creates turbulences due to the irregularities of the shores, so now there are two things:
- those turbulences might pull you under very rapidly, many meters below the surface, and that might cause you a shock. If you pass out when that happens, well... you're dead.
- those turbulences and the currents sometimes detach boulders or other chunks of rock, when this happens water needs to fill the void left by the fallen rock and if you are unlucky to be in that water it can pull you down for tens of meters. 10m of water on your head increase the pressure by ~1atm, if you get pulled under by 20/30m in just a second you will pass out for sure and as in the other point... you're dead for sure.
If you go to one of those lakes and want to swim you must stay close to the shore and you must enter the lake where the shore is not steep or else you'll be risking it.
Lake Garda, Lake Idro, Lake Moro and other are very much safe on the other hand. That's why asking a local person is often wise: it just takes a couple of minutes to find someone and ask, after you know you're safe get into the water and enjoy :)
On jumping in, you can get cold water shock, which can cause involuntary gasping and loss of muscle control, leading to immediate drowning. The water doesn't even have to be that cold.
