Abu, thank you very much for the detailed explanation. I've spent hours this morning trying to recreate your steps from the ground up so your efforts weren't in vain, but I fear I'm stuck at the stage of pasting the cut waves...it seems I'm not picking the correct Paste option bc the selection is pasted as a Floating Selection (see last image) and not allowing me to Invert the colors. I tried to Anchor the layer but then it merges with the Linework layer and doesn't let me keep a separate Waves layer.When I "paste as a new layer" I can't invert the colors either. Very frustrated bc I can't seem to get paste this rudimentary step. I also inserted the ocean layer as you said.

I figured I'd throw it back to you so you can look at the steps I took and hopefully tell me what I'm doing wrong.
These are the steps I took:

1. Open inked-in linework image > Rename layer to "linework" > crop image

2. Drag/drop parchment texture into Layers tab > Make bottom layer

3. Change "linework" layer mode to multiply

4. Add new layer "ocean" > choose blue for foreground > Bucket fill > Set layer to multiply >Opacity 75.0>
reposition layer above "parchment" layer

5. On "linework" layer > Free Select Tool > Select waves > Cut > Paste (Floating Selection (Pasted Layer))

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