Not a bad scene but It's really rather flat. I like the designs of the two girls in the this image, I think they're rather cute but I feel they're either too cartoony or not cartoony enough. Primarly my issue is with the hair, If you want the to be sylized more work needs to be put in to the hair to make it look cartoonish. I suggest the hair be modeled but that's just my suggestion for how to go about it.
Other than that the lighting needs work. There needs to be a extra lamp to act as a fill lamp. Something to really make these two pop out from the largely bland background. There's just too many harsh shadows, shadows doing nothing useful. Good lighting is half the battle of any good render. And with a good lighting set up I think that this could be really great render.
The scene composition could really use some work, there's alot of empty space in this. I say either move them closer togather or put something in between them. Maybe something like some drinks, some towels or maybe even a beach umbrella. Adding something like that could really break up the image and make it that slight bit more interesting and even narrative.
Now I'm not gonna sit here and pretend this is something easy to do but, They should really look a bit more wet. That would certainly make this image way better.
It's not bad but this piece is really lacking winning details. Personally I say keeping working on it. I think you could have a real winner here if you do.