For this, the best thing to do is use a modified AIR CANNON (see body/head hits). The neck of the hose would have to be a bit long, and the actual air would have to come out of the end of the hose instead of a small hose on the top. You probably couldn't fire very many at once, either.
To make an explosion underwater, just use a bigger hose with a little more air. it should produce a good looking explosion, and you could always make it look a little bigger in post.
