Either poured or blown sugar would be cool, but if needs be you could maybe cover the cake in clear piping gel to get a glassy effect. But I feel it might be kind of hard to smooth without messing up what's underneath.

Good luck and post pics!

Or, you could just use a plastic fish bowl like this --
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1168986