Can the H2shOw decorations be used safely in water?

Of course: they are made from resins, ingredients and colours that are 100% safe, and they do not give off any odour of solvents.

How do H2shOw decorations differ from other products available on the market?

First of all, for the quality guaranteed by Hydor, and the specific design with transparent parts or holes, designed for use in combination with the H2shOw BUBBLE MAKER and LED LIGHT.

I've heard of H2shOw "worlds". What exactly does this refer to?

It's a theatre for the aquarium, where the backdrop, wings, decorations, bubble maker and LED lights are all designed in line with a particular theme: Magic World, Atlantis and Lost Civilizations.