#1: composition is good, but colors look flat. the only way around that would be to take the same shot on a non-overcast day. the water would be blue instead of gray, for instance. note where you placed the horizon, 1/3 of the way from the top of the frame, to compare with later pictures.
#2: a polarizing filter would deepen colors, but what this picture really lacks is dimension and depth. photography is about replicating 3-D spaces in a 2-D image. you need a point of reference in the foreground to give it scale. my best analogy for this is two beach sunset shots that I took: both had beautiful skies, but the bottom edge of one shot's frame stopped at the first waves, while the other came all the way in to show sand; needless to say, the picture with a foreground reference point (the beach) was much more powerful.
#3: I like the colors, and the composition is alright. the only way to make the colors better would be an HDR image, which is exceedingly difficult with the ocean (identical frames of waves = impossible). I only called the composition "alright" because the horizon line is smack-dab in the middle of the frame. if that's what you intended, great, and I think it works fine here. if you go back to the same spot with the same lighting conditions and weather, I would suggest taking the picture at different angles to place the horizon 1/3 from the top and 2/3 from the top, just to see what it looks like.
#4: again, I like the colors, and I see what you aiming for with composition, because I make this mistake all the time. that dark band of ocean straight across the middle of the frame literally cuts it in two. if you were to crop away the bottom third and one sixth off of each side, then that dark band of ocean would act as a sort of underline for the sky, anchoring the bottom of the picture.
every photographer tries to take pictures of sunsets. it takes time to get good (and I need more time myself), and I should imagine nobody ever gets perfect.
#5: once more, I like the colors, and I see what you're doing. I would crop away some of the black areas around the sides and bottom edge, as it makes the picture very imposing, instead of the window effect towards the receding sunset. this would be an interesting candidate for an HDR, with all the pretty colors.