I have a project in 3D art and my idea requires that it be under water.?

What kind of clay or other means can I use? I have the pieces floating in a fish tank. Polystyrene spheres have to be used as a basis for some of the objects (which are light enough to float), but is there any kind of clay that can be used to cover that work well in water? Any advice would be appreciated as this is my first foray into the art of 3D.

treat Plasticine or clay oil, same thing really. it is oil based, not water based so it will not decompose in water. I think you can get it in colors too. that is what artists use Claymation TV shows like Bob the builder or Gumby,

Haliborange Shiny School, Chris Corcoran Art Masterclass – Paper Plate Fish, 4-6 years