The noun shoal can be used to refer to a group of fish or an area of shallow water. So when you’re navigating a shoal in your row boat, you might look down and see a shoal of fish swimming out of the way.
If you like to fish, you might know that shoal can refer to shallow water, but it can also describe a sandbank that you can only see when the water is low. Both kinds of shoals are problems if you’re trying to navigate in a boat — you have to try not to run aground when you encounter either one. The word also has a verb form that describes water that gets shallow: it shoals.