This ascender can ascend no more
You ascended from the water but now you are at your peak and can ascend no more. It's not cold enough to grow bigger.
Once feared, now fearful
Boats fear you, but now that you drifted to warmer places and the climate starts to change, you fear for yourself.
For I cannot overcome this roof-less, ruthless room
It's a sunny day in the vast ocean. You can't spot a single cloud. The sun is literally killing you.
This impossibly solid snow on which I can neither thrive nor survive
Even though it's getting warmer and you melt bit by bit, your internals are still solid! Unfortunately, there will come a day where they too will melt.
Am I to revert whence I came?
You're not so sure anymore that you will live forever as you thought when you were young. The waters you are now drifting in, could they have been big, proud icebergs just like you?
To the wettest of paths on the driest of days?
You realize one day you will become part of the ocean. Where even on sunny days like this, it will always be wet.