Ryegrass is a kind of grass, not the crop rye, which is why you can't gather it.
If you create a cave-in of the ice onto a "warm" (i.e. non freezing) surface the ice should melt. However, I think (without knowing) that doing this to murky pools or a stream won't work because the cave-in would also expose the rock below to the open, freezing air, so it still wouldn't melt until the surface melts.
A cave-in of glacier ice not exposed to the surface air will melt, however (provided you drop it on a "warm" surface, not more ice).
I'd take a two pronged approach:
- Gather all plants you can find on the surface by a blanket designation, rather than trying to cherry pick. Some of them ought be brewable.
- Dig down quickly to locate a cavern with water. Depending on the severity of the thirst you can have the luxury of building a safe(ish) pool/well embryo behind an airlock, or you have to rush out into the cavern to stave off the dehydration (make sure to make buckets!).
I'd complement the first one by browsing the plants marked for gathering, using the wiki to identify them, and bumping the collection priority on the ones that are brewable.
Given the ice, you either embarked in a rather cold embark, or it's already winter (which is about when you run out of the embark booze). You should have set up your farm plots, but if you haven't plump helmets are both brewable and will grow during the winter. That, requires you to have access to underground soil, of course.