Before you do that, I'm wondering if you should check a little further. I wonder if the glass cages (I'm assuming for a reptile) have the same level of water sealing that an aquarium does. I've seen some really cheap glass cages that have metal edges which connect the different panes. Salt water is pretty corrosive and can weaken the bonds.
Here is what I would suggest:
--I might fill the cage full of water in a garage or somewhere outside and make sure it is okay.
--If not that, I would at least examine the edges to make sure that seawater wouldn't corrode anything
--I would also compare the thickness of the glass in the cage against an aquarium. If it's the same, you are probably okay.
I think it's better to be safe than sorry. I would hate to come home and find that my aquarium dumped all of its water, fish, rock, and inverts all over my house while I was at work.