"Half Light" (2006)
starring Demi Moore
Rachel Carlson (Moore) is a successful American murder mystery author living in London with her five-year-old son, Thomas (Balawi) and her second husband, Brian (Cusick), a successful book editor that has been unable to get any of his own works published. His mother being too busy working on her latest novel to play with him, Thomas goes outside to play outside their canalside home, only to accidentally drown, devastating Rachel and putting a tailspin on her marriage and her ability to finish her latest novel.
You can read more of the plot summary here:
It doesn't show much of the opening incident, but here's the trailer:
I just found the complete film at YouTube!
Since you couldn't remember the rest, I can't be positive except that it starts with their son's death by drowning.