Create a new layer: Layer – New – Layer and represent on it an arbitrary ornament, making use of the Pen Tool and Fill Path command:

Designing Marine Wallpaper

Make a copy of this layer: Layer – Duplicate Layer (ctrl+j) and the ornament on it should be smaller a little. Turn it around with Edit – Free Transform (ctrl+t).

Designing Marine Wallpaper

Merge down the layers: Layer – Merge Down
and start using the Shape Tool (holding the shift button) to add several standard ornaments of different sizes (in this case we'll add the circles).

Designing Marine Wallpaper

Now we have to change the Hard Round brush's adjustments the way it is indicated in the tables below:

Designing Marine Wallpaper

Introduce with the same brush several wavy lines:

Designing Marine Wallpaper