Question

what was the purpose of sending the message about a water problem on Midway?

Answer 1

If this is WW2, it was to confirm that Midway was the target mentioned in deciphered Japanese communiques.

The US broke their military codes, but they weren't sure what the objective for an upcoming operation was. The US also knew that Japan were listening in on what they were saying, so the military sent a secret message to Midway about a water problem. When Midway reported the faked problem and the Japanese mentioned it in their codes pertaining to the operation, the US finally found out that Midway was the target.