Have you checked out the info on making snowglobes at my site yet? If not, here's a link:http://glassattic.com/polymer/outdoor_snowglobes_fountains.htm
It has a lot of info about those factors and discussions about what clayers have tried, etc.
As for chemicals "destroying" the clay, that mostly applies to anything in direct contact with the clay that has a petroleum-based solvent (those that must
be thinned or cleaned up with paint thinner/etc). Those substances may begin dissolving the clay immediately or eventually.
The main problem with polymer clay in continuous (soaking) contact with water, over a long period of time, though is a whitish coating that can appear as much as 6 months later. It will mostly show up well on the darker clays colors. That's caused by the clay absorbing just a tiny bit of water over time (and would disappear if the item could be dried out). Most of the clear finishes that can be put onto polymer clay are just the tiniest bit absorbent over time too.
Some of the most promising solutions might be using 5-6 layers of polyurethane (not Future in this case, btw), using liquid polymer clay as a coating, and using distilled water in the globe, but there are others that may work too.
(The inside of the globe can also display rot, algae, or rust if the materials inside aren't right.)
HTH and show us your results!
(Btw, silicone should be okay, I think...it's often used to adhere and seal the cap to the jar when making snow globes from baby food jars. But not sure about direct contact with the clay though since I seem to remember that some silicone caulks won't clean up with water and soap. Some of the acrylic caulks seem to be fortified with silicone though, so look for both the words silicone and acrylic if you see them on the tubes?)