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All right, harsh critique it is.

On the first drawing, what is the character doing? Is he casting a water spell/summoning water spirits? Or is he just doing a funky dance? In any case, he is at a very, very unstable pose. Even more action-type poses/dramatic poses have some amount of stability, either being stable at the moment or leading to something stable. It's either that, or the character is going to fall, such as in this case. His entire torso is behind in space in comparison to his knees; such a pose is only stable if he were sitting or laying down.

Draw the entire figure (loosely) first before settling on details. If it helps, do it on a separate piece of paper as a stick figure, in a small scale. Work out exactly what pose you're aiming for first before actually drawing the character in its finalized state. You'll probably figure out a stronger, more stable pose. Then, start drawing the figure based on your sketches.

For the merman drawing, think in smooth waves. For any subject you want to feel floating or swimming, just draw out one smooth, semi-wavy line and base the entire figure on that. On that drawing, I see a lot of straight parts, like his torso. It hardly bends with his tail or reacts to the fact he's swimming. The tail is well done, however, I love how it curves.

In the first picture... That part on his pants to the right seems weird to me how it's sticking out like that. It looks too low to be his hip. The hand to our left has a very awkward pose. I'm having a hard time making my hand look like that when I try, so I can only imagine the unlikeliness of him doing it by accident. I'd make the edge of his face to our left more straight after the cheek (i.e. beside the eye), though it could just be your style to make round heads like that. Lastly, I don't know what the first kanji on his shirt is, but I believe you drew the second one wrong. The long line on the left should squiggle a bit more... I know it looks like how you drew it on computer fonts, but I believe it's supposed to look like... oh, here's a site with a picture of it: http://www.kanjisite.com/html/start/rhsinfo/r_doumichi.html
Also, the thigh on our left looks like it's caving in or something. I'd have it bump out rather than in. Pretty cool overall.

The mermaid's arms seem too short, and the hand to our left is a lot bigger than the other hand. As Kupocake said, the tail is quite good.

Third picture, those hakama pants look like they're tied too high. The torso seems rather thick, even if it is a bit turned, or maybe it's just the head that doesn't match it. The neck seems too thin to me... I'd move the line to the left over more to the left, but I'm no expert on necks. The pinky on the scabbard looks weird... like it's longer than the ring finger. The katana is missing uh... I dunno what it's called, but it's a piece that goes on top of the guard. For wooden swords, it's a rubber piece that holds the guard in place. I assume it functions in a similar manner for real katanas... Anyhow, just look at a picture of a katana and you'll see it. The hand on the hilt is quite nice.

Fourth picture... The headphone to our right... That uh... inner part thing... The way the lines are curved suggests that we'd be seeing the surface of it, not just the side of it. I'd add another curved line to make the surface's shape clear. And is that the kanji for "1" or is it one of those lines used to draw out sounds longer? If the latter, you should have the line vertical since the writing is vertical. Here's a page from some raw manga:
http://i46.tinypic.com/2hobpqw.jpg
You can see it in the top-right panel (いやー) and bottom-right panel (はあー). And maybe his head isn't quite angled enough for his ear to be that high. Pretty good picture overall.