Ophelia’s will is doubtful here and this debate calls into question a lot of things. In Gertrude’s account, just moments before in the previous scene, Ophelia is the passive victim of her clothes. She falls in the river, floats there and is pulled to the bottom by her garments. At no point in this description does Gertrude suggest that Ophelia means to drown herself.
We’ve seen how Ophelia was before she drowned. She doesn’t seem like someone who is WILLFUL about killing herself. If she had been seen collecting rocks to put in her pockets, then maybe we could attribute her death to drowning. But no one reports such a thing and the way her death is described does not suggest she was carrying stones with which to drown herself.
It is entirely possible that Gertrude is fabricating this story to give Ophelia a more poetic end than suicide but even if Ophelia had an armful of stones in her mad scene – as someone not in possession of her right wits, as someone divided from herself and her fair judgment, there is still an incredible lack of WILL in any self-destruction. Her will was lost with her father. If she ever had any real will to begin with.