I masked part of the bottle, made new layer, and flipped it.
step 2 of 4
My personal source photo.
step 3 of 4
I removed part of the fish and created some guts with help of the tutorial mentioned above, removed the text from the fisherman's shirt & added mine. I used the brush tool to airbrush things like mouth parts, fish meat. and the creature eyes. Duplicated head layer for shadow applying gaussian blur and reduced opacity. I also tinte the creatures skin to be less human and closer to the seaweed color. It was a fun project.
step 4 of 4
I removed the remnants of the hat that was on the original guy, replacing with more seaweed.