Thank you everybody,
Monday afternoon I tried Mon Repos swimming pool. The swimming pool is wonderful

but it's real a mess

Nobody there respect the levels so there are people who swim fast swimming with people who swim slow...
Anyway I've seen that it is closed from 7th July to 26th August so I think I will try swim outdoors.
About swimming in the lake I'm not really sure. I get nervous when the water is dark and I cannot see what I have around me :S (yes, I should probably thank Mr. Spielberg and his "Jaws").
I will keep updated the post with my next try
Have a nice day!
M.G