If you float a candle in a cylinder and place this cylinder within a cylinder, it appears that the candle is under water. We used these for isle stands, and consisted of 5 James Story Orchids and a floating candle, oh yeah, and about 5 drops of red food coloring.
You may think this idea is cheap, it is once you own the vases, But don't forget all the work it takes to fill with water and consider teardown too.
Sometimes depending on the venue I will fill vases with water and use surrand wrap to seal vase. I always use double tied milk crates for heavy vases, well just about everything. And TONS of nylon zip ties, TONS.
I should take some pics of my delivery system and share with you.
And always never lift water filled vases from the middle, these vases appear thick, but they are actually paper thin. Always tilt and lift with one hand from the bottom and one hand on the rim. Trust me, I am talking from experience here.
I do have a question though.... Do you ever have a problem with the flame breaking the glass? As it is down inside another glass item??
I have some low glass bowls that have a slight lip and when I rented them with floating candles about half of them cracked from the flames being too close to the glass lip.