On one hand, I can't condone wanton harm to marine creatures. On the other hand, a dive knife that injects a blast of CO2 into into the target cries out to be tested on a pufferfish, no? Rather than a harmless watermelon.
Good to know that when the greased watermelon game gets out of hand at your next pool party, a wild melon can be easily daiquiri-ed with The Wasp. Divers commenting over at Wired are undecided if you could get a shot into a shark quick enough to save your leg.