Archimedes was a scientist that created a principle called "Archimedes' principle". This principle says that a body at rest in a fluid is acted upon by a force pushing upward called the buoyant force. Buoyant describes the ability to stay afloat. Buoyant also describes someone in a cheerful or optimistic disposition. This joke is playing on the word buoyant and its different meanings. 
Archimedes was buoyant at the thought .