We breathed life into Bertrand, but sadly, he didn't take flight. He tragically insisted upon beaching himself, and we ended up putting him out of his misery.
The idea was for him to be a solar hot air balloon, the air inside heating up and lifting him. I'm confident the main problem is too much mass of plastic in the pectoral fins, and tapered tail relative to how much volume of air those areas hold.
Holla to Make for featuring me. Thanks guys! If there's any interest, I could Instructibles my process in a little while.