matchstickman47
Fish Herder
you could look on the beach, bottle have been worn down
that's an awesome idea - i wish i'd though of it.
trouble with sea glass is there's not that mush of it, so it's hard to find and also, it tends to be in very small pieces - rarely larger than a square inch - so it wouldn't make good caves.
personally, i think the blowtorch idea is the best.

unfortunately I just did some research and I think melting the glass isn't an option. Due to it being so brittle, apparently you'd need to heat up the entire object very evenly as using a concentrated heat will cause that particular part to expand and will cause the glass to shatter..
I have thought of something like this for a living art piece. A community tank where all decor was trash. like pot, cans glass, a boot, broken plates, those plastic rings from soda cans, and just about anything aquarium safe.
I would call it "what our water ways really look like." It would be a statement.