The Rapture

First off, congratulations on finishing an epic picture. It's obvious that it took a lot of time, effort, and TLC. I think the end result was definitely worth the effort, and I imagine you learned a lot from it. That said, I have some suggestions that you might want to consider when you take on future pieces:

MOOD
The palette/color scheme seems at odds with the picture's contents. I can't say whether that is 'bad', but I can say that it doesn't feel intentional. You have these very bright neon colors, and then splatters of red that are inconsistent with the rest of the palette. Nothing about the palette or mood suggests 'creepy', in my opinion, though in theory, deliberately choosing a palette that is inconsistent with the contents of the picture COULD create creepiness. It just doesn't seem to have happened here! In the future, think about your palette in terms of more than just 'what colors do I like', or 'what color is this object', and more in terms of, 'what colors will make the viewer FEEL like I want them to feel'?

DRAWING
I see that some objects are drawn three-dimensionally, and make sense (the floating suitcase is a great example, as is the little girl). The little girl has some slightly questionable anatomy. For instance, the raised arm is considerably longer than the resting arm, and she is holding that stuffed toy pretty awkwardly. The neck looks too long to me, but I understand that many people elongate the neck intentionally, so I am not willing to say it is a 'mistake', but instead - that it looks like a mistake to me.

Good job on introducing elements into the foreground, like the bits of coral. They do a lot to make the image feel more three-dimensional. The weakest aspect of the drawing is - like you are already aware - the cityscape in the background. On one hand - kudos for drawing all of that, really. Few people have the patience and dedication to do so. It even looks like you tried to keep these in a style consistent with Bioshock, AND gave every building its own architecture. THAT is very cool. However, they are not at all on the same ground plane with each other. In fact, I can't even tell where the background ground plane might be, because the buildings are pointing in every direction. It is hard to tell whether the ground plane back there is supposed to be tilted back away from the viewer, or tilted forward. Whip out a ruler, man. Draw guides for all those buildings. It helps.

This book is really dated, and OH NO A CHRIS HART BOOK, but I swear by a lot of the info in it. Check it out starting on page 79 for some valuable info on perspective. This is a link, by the way. Click on these words.

COMPOSITION
This composition successfully gives the viewer the information they need, but not necessarily in the most effective way. Consider composing images to convey mood and add impact. Here is a link to Wikipedia's information on compositional techniques.

I would have personally loved to see this feel less like a posed snapshot, and more like you've actually captured the character in the middle of doing something. If it is supposed to be more about the character, and less about the environment/scene, more focus on the character may have helped (making them larger in the image, etc.). Having the back wall of the scene be flat against the picture plane also takes away from the potential excitement/impact of this piece.

RENDERING
This image has SOME hard highlights, but seems to suffer a lack of hard shadows. Additionally, while you clearly understand that light reflects, the location of the light sources are difficult to pinpoint. Some lights reflect onto the figure, while others do not, and no light seems to reflect any more than any other light, flattening the figure.

As for actual brush strokes, you seem to be doing this smeary sort of thing, where you try to get rid of the actual strokes. That seems to work well in the water, but poorly on the architectural elements. Smudginess is generally frowned on in digital painting, because it looks amateur/unfinished. Defining your shapes with more shadows may help you prevent things from looking 'mushy'.

Anyway, overall - it shows that you worked very hard on this. It has a lot of really nice details (look at those barnacles!), and I think you should be proud of it. But, of course, we can always improve on everything we do. The next big thing you do like this will be even better.