Your original text is quite clear, but here is a grammatically improved version:
"Herons prefer to hunt in shallow water. They stand very still and watch carefully. Once a heron spots a frog, a fish, or a bug, it moves closer slowly and snatches it quickly with its beak."
Explanation of improvements:
- Changed "the heron" to "a heron" to match the general statement about any heron.
- Added a comma after "fish" in the list to separate items clearly.
- Changed "their beak" to "its beak" because "heron" is singular.