Riva del Garda is a really nice place. I'll be there in July and in August in Torbole for two regattas. Really looking forward to it.
So regarding the legal I'm not familiar with Italian laws. In Germany you can shoot and publish pictures as you wish as long as individual people are either not identifiable or not the key point of the picture. IMHO this is the case in you last two pics for example.
Otherwise you need the explicit permission of the person photographed. For the first two pics for example you would need the permission of the kids, or actually their parents.
So that's the situation in Germany and I would venture a guess that it's similar in Italy.
Now for the practical part. Together with a friend I'm running two sailing blogs and we are faced with the same problems as you as we cannot ask every single person to sign a waiver before we take a picture. If I'm not participating in a regatta myself I talk to the guys in the race office and tell them who I am and what I do. If you ask, usually you can also get onto one a motorboat to take pictures (on a club level regatta this might be the race committee boat which stays at the start line or on of the boats which place the marks. On a larger regatta the organizers provide boats specifically for the press). If I then photograph individual crews or crew members I sort of take their agreement for granted as every sailor knows that the event might be covered by the press.
To be honest, I don't know if it is 100% legally correct. I never had any complaints but plenty of people asking me to send them pictures.
