Your paragraph is mostly great and clearly written! Here is a slightly improved version with correct grammar and punctuation:
"A beach day is great fun! You can lie in the sun and make sandcastles. When you are in the water, you can swim with dolphins and sea turtles. Watch out for the scary sharks and jellyfish that can sting!"
Explanation of corrections:
- Added "you are" before "in water" to make it a complete phrase: "When you are in the water".
- Added a period at the end of the last sentence for proper punctuation.
This way, the paragraph reads smoothly and correctly for a young reader.