Not exactly a sea creature, but close enough. I painted this in watercolor (original below) from a photo I found on Pinterest, then took it into Procreate to select the background. I had trouble getting the selection right, so ended up making a rough lasso selection around it and erasing around the edges. I duplicated parts of it and changed the hue and saturation and blending modes to make the fish more orange and brighter and painted in some eye details. I used a Splatter brush and another texture brush to add interest to the fish and bring in some white spots. I used your watercolor background, tweaked the color and then used the distort tool to make it a little more wavy like water. Then put another layer above that to make a lavender shadow. Because I erased around the edges, the fish looked like a cut out, so I put it on Alpha Lock and airbrushed around the edges. Still looked a bit like a cutout so I merged it all and used the Smudge tool  with the Soft Airbrush and lightly went around some of the edges to soften them. I am amazed at how much better this looked after editing in Procreate. Thank you for a great class!

My original watercolor below.