Here are the suggested completions for the sentences:

1. flows
2. The river is wide in some places but very **shallow** in others.
3. Look! There's something floating on the **surface** of the water.
4. They're building a harbour to provide **shelter** for the boats.
5. The boat had **capsized** in bad weather and one person nearly drowned.
6. Looking out to sea, there were a couple of large ships on the **horizon**.
7. It wasn't very nice on the boat because the sea was so **rough** and the water is about two metres deep.
8. We had to climb a **steep** cliff, which was very difficult.

Feel free to adjust any words to better fit the context you're working with!