I guess he just has little water tight capsules with NitroGlycerine or a thermite explosive so when the capsule hits the water the a hole in the side of the capsule leaks water in which causes a chemical reaction with the chemical and goes BOOM. Another possible way is to make them like mines and just call them depth charges anyway, you know, like a sinking mine. Take a capsule and fill it with Sulphuric Acid, Gas, Potassium Chlorate and sugar. The Acid must be concentrated, careful it is corrosive and toxic. The capsule when dropped should float just under the surface of the water. When it comes into impact with something it will go BOOM!!! That guy must be part of the Australian Battle Group.

Cheers,
Jimmy