The perspective on the organic features (jellyfish, kelp and to a lesser extent the whale) could be improved a lot. Right now they look like 2D sprites and have no sense of depth.
For example, the kelp growing on the sides of the buildings facing the street could use some foreshortening compared to the kelp that's seen straight-on (especially noticeable in the bottom left corner and top left balconies). The kelp also stops very cleanly whenever it reaches a line, making it almost seem painted on at some points
The perspective on the buildings is well done and they have a nice variety. However I think that the lines are too uniform and take away a bit from the abandoned feeling of the city. This may just be a personal opinion, but I believe that using the straight line tool more sparingly and not making every line in the same width could help add more subtle imperfections (this also ties back to the kelp thing, you could paint over the lines more often to make it seem more realistic). Doing it like this might save you time too, you don't need the lines to be 100% accurate to the perspective in order to produce a sensation of depth.
Finally the lighting does not really suit the underwater theme. Right now the lack of shading makes it look like it could be a city in an overcast day, while in a large underwater scene there would be a decrease in brightness at all points at lower depths. A yellow glow seems to surround the edges of the picture, which no sense to me (although even if it doesn't make logical sense, you can still use things like that for a nice effect, I just don't like it here much).
The jellyfish and whale could also use some slightly more pronounced shading, as this is crucial in producing the sensation of volume that they're missing.
Overall I think you have a good understanding of conical perspective, and I found the building designs to be pretty original, but still believable. It's also refreshing to see a picture with more muted colors on Newgrounds, and it has a pretty original theme. Just keep practicing on organic shapes though.
Sorry if this went on a bit too long, I don't mean to be presumptuous or anything.