I would change "down the river" to "into the river".
Please have a look at the paragraph. Thanks for your kindness!
Last Sunday afternoon, while I was walking along a river bank, a boy was swimming in the river in the sunshine. I originally thought that he was just frolicking in the water, but he was sinking. He shouted loudly for help, so I immediately plunged down the river to save him. When he was helped ashore, I told him, “Don’t you see the sign ‘No Swimming’ over here?” “Sorry, I didn’t notice it,” he answered. After my blame for his fault, he promised to be more careful and would never do it again.
I would also change "Don't you see" to "Didn't you see."
I would use "Didn't you see the sign?" because it really doesn't matter if he can see it now. The question logically queries whether he saw it before he decided to jump in the river.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
In the sentence, "When he was helped ashore,...", is the phrase "be helped ashore" correct?