Well it can be a little obvious sometimes with the consistency of markers(obviously seen with the attempt in the background) but it still keeps with the various characters. The mermaids nose could use a little work as the shading tries to make me think that it's a more realistic nose while the shape drawn hints more of a cartoon like one. Based on the shape of the nose it might have been better to avoid shading all together or just faintly(appears too dark to me)
I'm not entirely sure why there is a flower bubble in the scene(or maybe it's a starfish trapped in a bubble?) but it doesn't really seems to fit almost like a random fly on the paper.
Other than the minor critiques above; the remaining features of the picture are drawn rather well and maybe the white octopus's suckers could use a finer stroke to highlight them while some almost seem a little too squarish.
Overall this picture captures the viewer well and is rather impressive considering it was largely done with markers.