Corrected sentence:
Rescue machines are always ready to help. Whether on the ground, in the sky, or on the water, they will find you.
Simpler version (even easier to read):
Rescue machines are always ready to help. They will find you on land, in the sky, or on the water.
Why this is good (easy steps):
- We use two short sentences so it is clear and simple.
- Using "Whether" shows the three places as choices: ground, sky, or water.
- We put a comma after the introductory phrase (the "Whether..." part) to make the sentence easy to understand.
- "They will find you" is correct — the verb matches the subject "machines."
Original sentence was mostly okay, but these small changes make it clearer for young readers.