Probably too expensive for execution of criminals in general, but I like the idea of dropping them out of a high-altitude airplane over the middle of the ocean. 

Death, when it comes, is quick, clean, humane, and completely painless.   Terminal impact on the water literally happens too fast to feel physical sensation before the brain is destroyed.  But the condemned would have a couple of exciting minutes on the way down, to spend either screaming in terror or making peace with God, as he prefers.