So - I finally finished the sandcastle cake. There was a good deal of seaweed but no black bubbly things (I think those are egg cases for some sort of sand shark). The kids had a great time at the party and when it came time to have cake and ice cream - they ate all of the cake decorations. They weren't too interested in the actual cake. They were thrilled that they actually got to eat "rocks".
Lessons learned from this include:
Fruit roll ups dissolve into something that looks like swamp tar when the humidity level is high.
You CAN eat those realistic looking candy rocks.
Gummy fish stick together nicely when the humidity level is high.
Kids will even pass up chocolate if they think they can eat rocks instead.