How exactly do private beaches work in Lido, Venice?

In Lido, a private beach is only owned by the person, that's why it's called "private". It doesn't mean that you can't go on that section that is owned. You aren't supposed to, though. I think it's just called a "private beach" because someone owns it. And yes, the water cannot be owned. To use a private beach, the person or organization/company that owns it (eg. a hotel) may expect a few Euros for rent to use the beach, maybe for a party or something. That fee would cover the use of the beach, that's it. Otherwise, for public use, there shouldn't be any fees.