Why is punctuation so important?

Read the following for background information.

The dark blue lake was deceptively calm. Everyone in the community knew that it was a dangerous place to swim. The water was surprisingly deep; the current was always strong. In 1986, a six-year-old boy who was on holiday with his parents drowned. In 1990, a family stopped for a lakeside picnic and a swim. The entire family drowned.

The townspeople believe that the lake was haunted by evil spirits. They decided to create a warning sign so that no one would ever again swim in the lake.

The first draft of their sign read:

Private?

No: Swimming Allowed!