I don't honestly think that
Gollum intended to die with
the Ring, but here is what I think was going on:
Gollum was tied to the Ring because of how long he had taken it into his possessions. And so because of that, his need for it was exponential, and his desire for it surpassing even that. I don't know if Gollum would have died instantly after the destruction of the Ring, or if he would have survived a bit longer, but I firmly believe the only thing still suppressing his age and inevitable death, was the pursuit of the Ring.
It was
the one thing that actually gave him something to live for. We can see throughout the other chapters, his appearance, his solitude, Gollum had nothing more than the Ring and his desire for it. He did not have ornamental clothes, or any form of family, or even a home. He simply had the Ring, and when even that abandoned him, he was left with his desire for it.
When the Ring was destroyed, it would be the end. It was the only thing that Gollum would have ever desired or hoped for, and the only thing left for him to live for. I don't think it was necessarily as much the
evil of the Ring that preserved Gollum from death, but maybe the fact that he had not fulfilled his purpose. Once he did, he died. Instantly.
And while, it happened due to his fall into the Cracks of Doom, that was still his end. I believe that Gollum would not have sailed away, because he would not have been alive. Most likely, with nothing, he would succumb to grief, mourning the loss of his '
Precious'.
Or dare I say, he would have cast himself into the fiery chasm, in hopes of finding the Ring.
While it seems Gollum takes a mis-step, and accidentally is cast into the Cracks of Doom, I might even argue that a subconscious part of him, tied to the Ring and his love for it, purposely made a leap, in a false hope that he would have his Precious forever with him, and no one could take it.