I really like your idea and composition, and I really like her intense gaze. But I also think the wave breaking in the background confuses things for me because I can't make out the person properly (too much white on white).
Awesome photo. I love the excellent use of depth of field here, and the nice contrast - not too much, not too little. I do wonder about her expression though - it looks like only one kid is having fun at the beach. Or is that more of a Mona Lisa smile she's flashing? Very nice work.
Beautiful everything. I understand/agree with the idea of some background activity. The only change I might suggest would be to have the 'runner' heading out of the image. That might create a bit more 'pull' between the subject and the runner.