Change Its (possessive form of it) to It’s (contraction of it is).
The noun summertime is one word.
Add a comma after summertime (sentence is compound).
Best friends is two words.
Add a comma after You dive in (sentence is compound).
Add a comma after air.
It’s summertime, and you’re swimming with your best friends in the lake near your house. You dive in, but when you come up for air, you feel the chill of winter, snow covers the ground, and your friends are floating dead next to you.
A compound sentence may be more than just two simple sentences connected by a conjunction. The second sentence in this example is compound, and it could be broken into four shorter sentences by removing the conjunctions: You dive in. When you come up for air, you feel the chill of winter. Snow covers the ground. Your friends are floating dead next to you. That’s why the commas after winter and ground are correct.