Gonna drop a comment here for updates, because we're in a similar predicament and I'd love to see how you guys are doing! My mother bought some snowglobes at an auction as a present for my cousin and uncle, but the glue holding the figures inside had come loose, making the rest of the water really murky and gunky.
We've been doing some research on how to fix them... some snowglobes are put together with plugs, like pony heads, but not all of them apparently. Also, don't use pure water inside, but add some glycerin too. Apparently, you can find the stuff at some craft stores, but we've been unsuccessful. Glycerin acts as an antifreeze and will prevent the water inside from freezing, and it may or may not also protect it from algae. To minimize that problem, keep it out of direct sunlight.
If you want to save the snow inside, a trick I've read is to dump the water slowly, saving as much of the snow/glitter as you can. Then, when you've got just a little puddle on the inside, use a paper towel as a filter to collect the rest of the glitter. Let it dry, and then you can just dump it inside when it's dry and ready.
Good luck! We haven't attempted any of this yet, but this is all still some really nice "theory" that we've found during our research of trying to find a way to repair ours.
EDIT: I guess I got a little ahead of myself and somehow missed Karma's advice.
![Tongue :P](https://mlparena.com/Smileys/default/tongue1.gif)
Which is very good advice, so thank you as well, Karma!