Two soldiers are in a tank...
Two soldiers are in a tank. One turns to the other and says: "Blub Blub Blub!"
This is the reverse of the goldfish joke that I told a while back. This joke it not quite as funny as the goldfish version, however, since it has fewer layers of humor.
This joke is another example of a pun. The initial assumption of the listener is that the soldiers are in a military tank, however after hearing the punchline the listeners are supposed to realize that the soldiers are actually in a water tank (or perhaps a fish tank).