What was Ophelia doing by the water in Act 4?

Ophelia was hanging flowers from the willow tree when she fell into the water. The weight of her dress pulled her under, which resulted in her death. Note, Ophelia did not attempt to free herself from the brook..... so far as we know. Her death, however, does raise other questions. Did Ophelia allow herself to drown, meaning she commited suicide? Or did Gertrude know more than she was letting on.... why didn't Gertrude stop or help her if she was there and aware of what was happening?