Is there a specific reason why you took the dark shade for the killer cacti-guy's pricks/needles off, and replaced them with the lightest green again? The light green specs tend to blend in with the bright background
I also agree with the Fry-man. The castle stage looks the best so far. And if you won't animate the knight fellow, then I will.
As for the underwater Wip: I really enjoy the U character in a scuba suit. Add's a sorta' mini-realism factor to it. I would suggest putting two types of bubbles: one for the active characters (sprites), and some background bubbles that zig/zag upward. Also, the lines that represent the farthest reef/underwater cliff seems strange. I would try to add some slight dithering on the furthest reefs(I know there's a specific word for underwater ground and cliffs. Is it trenches?). The light shining through the ocean needs work to be honest. I would try lengthening every other beam of light in the tiles, and shortening the other beams( hope that makes sense
) Ahh and seaweed in the background. Just a teeny weeny bit of it. Not to much (maybe two seaweeds in the background would do the trick).
Despite those suggestions, this is looking uber simple-awesome. I wants mo'!