First two things come to my mind are "Moby Dick" & "The old mad and the see". I should notic one thing - on foreground, there are big waves, but in the background, the see/ocean is calm. Despite this fact, this pixelart made BIG impression on me and I rate it 5/5.
@ Ego: thanks for your detailed critique. I particularly appreciate your words on the whale AA. I'll do an edit asap. I'm also glad you noticed how the mariner is not at all bothering about the whale :) but bear in mind that he is still an apprentice. he's too busy trying to keep control of the boat to notice the huge whale behind; even if he should!
I think Ego's points are spot-on About the AA stuff. This is a very nice composition overall and I think the waves especially are rendered quite well.
This leaves a nice initial impression, but a closer look reveals some flaws in the pixel work. Despite the things I'm about to point out though, it definitely leaves a striking feeling - it just needs another stage of refinement.
As a first, very simple point, you don't have six colors, you have seven, none of which are near-clones so it's not just a matter of a stray pixel or anything. Not a big deal, just good to know, especially if low-color was a goal.
The biggest issue is the anti-aliasing; you went way, way overboard with it. Especially on the bottom left of the whale, it's just excessive. Consider for a moment what anti-aliasing is meant to do: buffer the space between two significantly different colors, essentially acting as an extension of the cluster. In this case, instead you have what amounts to a whole cluster of grey in between the background - it has ceased acting as anti-aliasing and is now it's own part of the shading.
I understand what you were doing; it's a pretty common use of anti-aliasing to try and smooth out the imperfect curves.When taken too far though it instead has this blurring effect - the visible manifestation of the problem. Even zoomed all the way out at 1x, look at that bottom left of the whale. You can clearly see that's a fuzzy transition, not crisp or clear. Same along the top curve. Over-anti-aliasing is directly related to banding, which is no good. Speaking of banding, don't anti-alias large spans of 45-degree angles, it just becomes banding. This can eb seen on the right side, on the body under the flipper.
Instead, work on making the main curve as smooth as possible and just augmenting it with a little AA. To illustrate what I'm talking about I did a quick repair job on some of the AA to show you what I mean: 
Now, even beyond the whale there's a couple issues. while you took the time to use the AA to try to smooth out most of the whale's curves and most of the lines in the waves are reasonable, there's a lot of fairly sloppy lines on the boat, especially at the base where it contacts the waves. The sailor's shadow color blends in with the purple wave color. I really like the waves' general form, they give a great impression and feel right, but again there's some confusion with what exactly the waves near the front are doing - it's not too bad though, and the waves around the breaching whale are awesome. Sompositionally, it feels a bit odd that the sailor isn't reacting at all the the whale - it feels like two different pieces that just happen to be near each other, rather than a single cohesive presentation.
Hopefully this helps you a bit! Sorry that this is a pretty dense block of text, I really do like the piece overall. If you need any clarification on anything, just let me know. Cheers!
No problem! And that's a good point, but in that case I'd say that it still doesn't look like he's particularly struggling - he's sitting there, just holding the oar, looking straight ahead. It's overall a minor point, but making his pose more dynamic could add energy to the piece.
The calm ocean/wavy ovean difference is a good point, but personally I like the visual effect because it gives that near-perfectly-rectangular negative sqace.