Donate to the Lead Adventure Forum to keep it alive!
Upholstery foam soaked in polyfillerThat is what we used for our aqua naughts
There is a build thread for the game but I can't find it at the moment
Another material you could use to represent coral is some actual sponge, soaked in PVA or paint.
Excellent idea. The structure of the sponge should hopefully emulate coral nicely. Sponges are cheap, so I'll have to give this a try
That is just the same method we used for our sea bed bases