Ophelia's death is appropriate for her character. She is paralyzed in a man's world, caught between her love of Hamlet and her duty to her father. Both men let her down. When Hamlet kills Polonius, Ophelia loses both her father and the man she loves. As a result, she cracks. She has an emotional breakdown and truly goes mad. Whereas Hamlet only pretends madness, Ophelia completely loses her sanity.
As she goes to gather flowers for a garland, the slender reed that she climbed upon could not support her weight (much as the men in her life provide flimsy support), and she falls into the brook. Not having the wits to save herself, her garments pull her down, and she sinks. So the death is partly accidental but caused by a paralysis of spirit and wit--and quite representative of her actions earlier in the play. In spite of herself, she had become caught up in the "corrupted currents of the world," when she helped Polonius spy on Hamlet.
If it is suicide, it is a passive suicide. The priest at Ophelia's funeral believes her death is a suicide, but Laertes does not. The gravediggers also debate the question. Just as many actions in Hamlet are difficult to judge, Ophelia's death is no exception.
I would have to say that I do not think it is suicide. At least, not in the sense where she is intentionally trying to kill herself.
If you look at what Gertrude says, it is not as if Ophelia runs down to the river, says "I'm ending it all," and dives in. She is climbing in the tree to hang up a garland. The tree breaks and she falls in. It seems like a pretty elaborate way to commit suicide.
Now, why doesn't she get out? I think it is not clear. You can say that she is insane and does not really realize that her clothes are going to get all heavy and pull her down. By the time she does realize it (if she does) she's weighted down and can't escape.