definitely Vidy, either swim from the beach near the basketball courts (free) in the direction of ouchy or from the pool at bellerive (5CHF) towards vidy. There is a marked section with yellow bouys. 450m long, 50m between the posts.

in fact it's more near the mini golf than the basketball courts, and if you talk nicely to the mini golf staff, they let you use their lockers for free.

best place to swim in the lake is the middle 

Really I'm fear of the lake-monster living in the depth of 310 m in the middle of the lake! better swim near the coast

I know the best place to swim in the lake, but its my secret!  

So, otherwise you can try Rivaz - one of those places you see from the train and wonder how anyone got there.

There's a sweet beach by Chateau Chillon, but it gets a bit crowded.

Another fab place (and I will post summer evening swims soon) is Lutry as they have a lawn, bathing area/steps, stone beach and a diving board (the only way to go in!).  For after swims, there is also a lovely cafe serving cheap food and beers, open until late evening (21h00? - not sure)..

Just after the pool in Villeneuve, there is also a very nice area where you can swim.

Bellerive (mentioned above) in Lausanne is fantastic and the 5 francs is well worth it as you can spend all day in there

Only rule of thumb is to swim East of Lausanne.  The sewage works spill out at the IOC building in Vidy and it starts floating down to Geneva from there on (well, we had to send our shit somewhere - and Geneva seemed the obvious choice!). 

wow, thank you Carolyn! A lot of possibilitys to try out!!

Uh, no, you got the wrong impression there, I'm afraid.

I'm not swimming TO the middle .... That would be sport !

Take a boat, go to the middle, jump off the boat 

The monster hates boats, you're in no danger ! And the water is really, really clean and just amazing ! No algae around your feet, no .... well, you know what I mean ...

How far can you be tortured -or maybe "rendered" to Syria - before you give up your "secret place" ??

Lac Neauchatel is very nice and clean compared to Lac Leman

Someone told me that if you swim in the lake, you have to be careful of duck fleas, and shower immediately after getting out of the water. Was this person having me on, or is this a real thing?

I believe when the water gets very warm later in the summer you can get a lot of duck flees in the still water at the edges of the lake which can cause a lot of irritation.

From 20°C you could get a few but in August, if the water temperature gets above 22, which it normally does, then it's imperative to shower when you get out. Do a web image search for "Puce de Canard" so you know what to look for. They are mainly where there is lots of weed (and ducks).

I'll second the "middle of the lake with boat" recommendation!