It is not clear that Ophelia actually committed suicide. She had become mentally deranged after the murder of her father by the man she loved. She accidentally fell into a stream and made no effort to save herself. Eventually her clothing became so saturated with water that they dragged her down.
I believe the main significance in the death of Ophelia is simply that it is just another illustration of how Hamlet's indecisiveness and procrastination lead to the deaths of many others. If he had killed the King, then he wouldn't have killed Polonius. If he hadn't killed Polonius, he wouldn't have incurred the enmity of Laertes and the mental breakdown of Ophelia. By allowing Claudius to remain alive, he gave him an opportunity to try to have him killed by the English and later try to poison him. His mother drank the poison by mistake. The deaths of Polonius, Laertes, Ophelia, Gertrude, even of Rosencrantz and Guildenstern, as well as of Hamlet himself, were all caused by Hamlet's failure to act decisively. His fecklessness was due, according to Coleridge, to the fact that he thought too much.
I think that Ophelia's death is significant on a dramatic level and a social one, as well. On a dramatic level, the state of affairs in Denmark have definitely run afoul when someone as innocent and as non-politically inclined as Ophelia has to commit suicide. Ophelia lacked much in way of political agenda and really had little in terms of of her own personal ambition present. Yet, the perverse and twisted condition of the Denmark Royal Court cannot be stopped by Ophelia's purity and innocence. Her selflessness is manipulated by those in the position of political power and representative of the human cruelty and revulsion on display in the Denmark Court. In Ophelia's death, a rather stunning statement is made as to how horrific things actually are in the kingdom. On a social level, I think that Shakespeare, whether intended or not, has made a statement about the condition of women during the time period and, perhaps, an applicable one today. Ophelia is selfless, willing to devote herself to the words of the men around her. She is loyal to her brother, loyal to her father even when he does not display the same to her, and loyal to Hamlet, who is incapable of being loyal to himself. Yet, her obedience and willingness to subjugate her own identity to those of the men around her cause her to end up committing suicide. Ophelia is an example of what happens when women's voices are sacrificed without much in way of care and reflection by men. Ophelia becomes the representation of why the assertion of women's voice is essential in a world where men are in the position of power. Her suicide is a reminder of what happens to any human being when voice is silenced and identity is not validated.