I recommend reading this blog post by Larry Correia:
As to your copyright question, if you're not blatantly taking characters from copyrighted works and using them in your own (as in fan fiction, or by copying them exactly except changing their names) don't worry about it. You've come up with characters that may be superficially similar to copyrighted characters in some respects, but that is not plagiarism and it's more or less inevitable in a world where millions of stories have been written and told.
This is another article, outlining what copyright law covers in the first half:
What it boils down to is story concepts and ideas are not covered by copyright law. Excerpt:
A copyright does not protect story ideas, concepts or themes. Such elements are not protected whether they are in a writer’s head, written down on paper, or published. Ideas are as free as the air. Ten authors can write a story about a doomed romance between lovers from different backgrounds. This could be Romeo and Juliet, or West Side Story, or another variation on the theme. Likewise, multiple authors can write biographies about George Washington. Each is free to tell the story of George Washington’s life in the writer’s own words. Each can borrow facts and historical incidents mentioned by prior authors.
What copyright law protects is the “expression of the author.” This is the particular manner in which the writer tells the story, his approach to the material, his voice. In other words, what is protected is the embellishment on the idea, not the idea itself.
The idea of a character being able to control water is too generic to be copyright-able. And, it's been used in probably thousands of stories. So don't worry about it.
The story ideas you've mentioned so far sound pretty cool, by the way.