How to add a square grid as a layer?

[swiss]

So I know how to add a basic square grid in Photoshop, but this is on top of every layer and is not its own layer. I have seen maps where the grid looks like it's its own layer, because the grid is more visible on the sea layer than the land layer, so I'm guessing they have a layer that is only a grid and then they can modify it from there like they modify every other map element. So how do you do that? The only thing I can think of is creating a new layer, getting out my line creation tool and literally just drawing a grid, but there has to be a more efficient way.

Here's an example of what I mean: https://www.deviantart.com/picantese...-Map-563638624

You'll see the grid is at least below the black coastline, so it must be a layer of some kind. It wouldn't be the standard 'add grid' in Photoshop