The colouring of the skin is flat, you have to add dark colours while using hard edge and soft brushes. the fish tail looks completely out compared to the rest of the picture maybe you should have done a bit of filtering with the the fish scale textures so it would have blend in or better, draw the scales your own way, not only scales are a good lighting/dark colouring practice but at least this way you can control wether it'll go with the picture or not, plus additional practice~
The background also needs help! The water looks more like a jelly than an actually water, I suggest looking at a picture of a real water ripple to see the diference, water ripples are like small hills in water and colour contrast are very harsh, it goes light to dark rather quickly like the one here.
The sand is okay but usage of a texture would be good, now onto the sun... Okay, that sun is far too big,why isn't the Earth on fire?! You might wanna rethink about the position, it's too near and big. You might wanna just remove it off the image since the composition of the image is small not large anyway,jumping back a bit, since the sun is in the direction use the position as a guide helping you put the shadows in place.
Now, the anatomy needs help also, the hands are too small for the girl and her arm reminds when Harry Potter lost all his wrist bones back in second year of Hogwarts and had to regrow them.
tl;dr that arm does not have a single bone. bones works like a mechanics, remember that.
The torso lacks muscles and bones as well. This mermaid of yours is slim, therefore her
hip bones and rib bones actually do show, plus the breast are far too perky and sharp. Mind you,
boobs are like triangular based cones place on a flat surface. And the last thing...where's her belly button and nipples?
Now that's all and done with. I must say, I love, love
LOVE the tattoos! They look great!