Please be considerate enough to use a "swim nappy/diaper". One "accident" in the fountain can make several people sick. You wouldn't want that for your child, neither does anyone else.
I think what you are referring to as a fountain park is comparable to our splash pad here (in the US). Unlike a pool, the assorted water toys are supported on a cement pad and the water is recirculated as it drains off the pavement. In all likelihood, the water does contain a certain amount of chlorination to prevent the transmission of disease, but... is that sufficient to think it would be okay to let a child who cannot yet control their bladder/bowel to play freely without attempting to prevent the spread of germs?
Generations of families have diapered their children to run and play in pools and at the sea. That was then. This is now. They didn't have things like Swim Diapers to help keep the water clean and free of diaper solids. Of course, we didn't know what E.coli was back then, either.... After some research on Swimming Pools and Health Risks, it looks like even Swim Diapers might not offer the protection we want. For our children, and others.
Every parent/guardian/caretaker should do everything they can to make sure that their child isn't exposed to dangerous elements, and to make sure their children are not making others sick as well. I'm not sure about "rude", but I would call it "inconsiderate".