Why, according to the second clown, is Ophelia really being given a Christian burial?

It is generally accepted that Ophelia has committed suicide. At that time, suicide was considered a sin, and a very serious one at that. Those who killed themselves were therefore denied burial in consecrated ground.
In act 5, scene 1 of Hamlet, a couple of grave-digging clowns are preparing the recently departed Ophelia's final resting place. They engage in all manner of wordplay and witty badinage to lighten the oppressive mood hanging over the scene. The gravediggers argue whether Ophelia will be given a proper Christian burial, being as how she's committed suicide. One of them comes up with a novel explanation of Ophelia's death, one that can be construed in such a way as to make it seem that she didn't intend to kill herself after all. He says that Ophelia didn't drown herself; the water drowned her:

If the man go to this water and drown himself, it is, will he nill he, he goes. Mark you that. But if the water come to him and drown him, he drowns not himself. Argal, he that is not guilty of his own death shortens not his own life.

In other words, according to the gravedigger, Ophelia didn't go to the water; the water came to her. As such, she didn't really commit suicide, and so can be given a good Christian burial.