Seabed in a Bottle
You will need a plastic bottle that you will fill with water and add a few drops of blue (i.e. cake colouring, tempera.)
If you have small figures of sea creatures, insert them in the bottle or cut them into shapes, such as stars and fish from the material that will remain unaltered in the water (i.e. flannel, foam.) Add shells and gel balls (water beads or gel beads).
Just in case, use super glue or silicone to tighten the cap.