I know where the water meter is. I need to dig a post hole near the water
Meter, but I am worried about hitting the water line that comes into the house. People have told me the line is only buried 12" below ground.
And to just go slowly and easy with the post digger tool. It is only one post. In live in south Florida

In most areas you are required to call Miss Utility (might have another name in your area) before digging . Someone will come out and mark all utility lines. You are only putting up a fence, but why take a chance of having to repair a pipe.

Where do you live, water pipes are always below the frost line, 48" in my area.

I know where the water meter is. I need to dig a post hole near the water
meter, but I am worried about hitting the water line that comes into the house. people have told me the line is only buried 12" below ground.
And to just go slowly and easy with the post digger tool. It is only one post. In live in south Florida

What's your problem? I live in SouthWest Florida and all you have to do is take a shovel and dig down a few feet and check to make sure it's clear.
Then sink your post. Good luck, Tom