At the end of Solaris we have Kelvin futzing about at the ocean and thinking vaguely spiritual thoughts about hope's renewal. So, water = rebirth. Again. Thanks, Lem. How many times have we seen that one? Approximately a trillion, give or take a billion.
But before you close the book muttering, "Ho-hum, more water imagery," consider this: That ain't water. The liquid Kelvin is mulling in is some sort of semi- or super-sentient colloid thingamabob. It's not your grandpa's Old Man and the Sea ocean we're asking for rebirth here—it's some sort of bizarre other thing with which "The entire human race had tried in vain to establish even the most tenuous link." Rebirth is figured not as the same old same old, then, but as something that looks like the same old same old but which actually no one can really understand.
So the ending note isn't so much, go into the water and be reborn or at least somewhat less dirty, hallelujah. Nope, it's more like wander into that whatever-it-is and hope for the best, sucker. Good times.
This is a metaphor for religious faith, in part. "Must I go on living here then, among the objects we both had touched, in the air she had breathed?" Kelvin laments in a state of grief. And when he asks, "In the name of what? In the hope of her return?" he's connecting the weird Solaris second-body rebirth to a rebirth in heaven.
So instead of making a boring water-rebirth comparison, you could see the ending as complicating that link, and thinking about the alien-ness or weirdness of faith, the way we look out into the distance and dream some dream is coming back to us that is not too foreign to hold in our hearts, to make sense of. When Kelvin says "hope of her return," it is fair to think of Jesus—either his resurrection or his second coming—and, in doing so, to recognize just how otherworldly both of these concepts are.
The ending may also be deliberately self-referential. The alien liquid here is not water and rebirth; it's a metaphor for water and rebirth, a weird ocean standing in for the real ocean. And that's exactly what happens in the usual ocean metaphors as well: They're not real oceans either, just words on pages. "That liquid giant" Kelvin's talking about isn't the ocean; it's something like the ocean, which is to say, it's words.
When Kelvin wishes that "the time of cruel miracles was not past," you can see him as lamenting the fact that the book's over and he won't see Rheya again—or as urging you to turn right back to the first page, so he can do the whole thing over again.