Everything is edible - even the coral reefs!
The coral reefs are poured sugar: Melt granulated sugar in saucepan, and heat to hard-crack stage. Add gel food coloring (NOT water-based!) a drop at a time until you've reached your desired color. Then, pour the hot sugar into a container filled with ice (choose the shape and size of your container based on how big you want your reef, and choose the size of your ice the same way). The ice hardens the sugar, and when the ice melts away, you have a completed coral reef! Be careful when you handle them - they are very delicate!
The diver is modeling chocolate - he is supported by a wooden dowel that we tried to hide amongst the sea grass.
The blue part of the ocean floor is cake covered in blue buttercream with blue piping gel accents. On top of that is brown sugar sand, and our multi-colored coral reefs.
All of the fish are chocolate that we colored and poured/painted into chocolate molds. There are also various types of seashells and starfish that we made with chocolate and molds as well. Finally, we hand sculpted and painted a sea turtle (far right) and a shark (far left, partially hidden from this angle) out of fondant.