3 You see a sign which reads 'No Swimming' and your friends are getting ready to dive into the water. What do you tell them?

Я бы сказала:

- You must not dive! It is dangerous. There is a sign here 'No swimming'.

Перевод:

- Вам нельзя нырять! Это опасно. Здесь знак 'Не плавать'.

Ответ: You must not dive! It is dangerous. There is a sign here 'No swimming'.