Having looked into this a bit, it would seem that the 'depth charges' found among the wreck site are infact Hedgehog Anti Submarine Morter Rounds. This changes the story a bit and should help to dispel the conspiracy theories abound.
The Hedgehog Anti Submarine Weapon wasn't developed until 1941. It's purpose was to throw a pattern of morter bombs ahead of a ship so that an the enemy submarine could be engaged without affecting the performance of the attacking ships sonar. The sonar types in use during the second world war used an operator listening for the acousting 'ping' to return from any underwater contacts. At close range the ping would return so quickly that range and direction would be almost impossible to determine. Submarines within this range would out manouver attacks and reduce the effectiveness of the deapth charges. Also, depth charge attacks required the attacking ship to work out the depth of the submarine, required the attacking ship to run over it's target before dropping a pattern and until the water settled after detination would almost always causes the loss of sonar contact allowing the submarine a chance to get away. With the hedgehog, the attacking ship could almost creep up on the target, maintaining sonar contact the whole time and it didn't matter what depth the submarine was at because the hedgehog morter exploded on contact.
This brings us to the wreck itself. The hedgehog, as I said above, was developed in 1941 but didn't see wide spread use until 1942. Initial performace of the new weapon was found to be about the same as that of depth charge attacks. As a result of this, a widespread training programme was initiated n 1943 which saw a huge increase in the effective use of the weapon against U-Boats. There were two types of round used with he system. The High Explosive round for operatioal use and the practice round for training use. I believe, and this is just my speculation now, that the wreck of the Lusitania was used by ships for training for the exact reason Michael gave, it provided a strong sonar contact at a known location and depth which would have been ideal for putting crews through their paces. Given the cost, and use of high expoives, which would have been needed for use against the enemy, ships would have used practice rounds for training which would have contained the charge for launching the morter and possibley a small charge in the fused nose to signal a weapon had made contact with the target but NOT the full ammount of high explosives. Given that the front end of the morter contained a contact detonator the operational HE version of the morter would detonate on hitting something solid. If ships were using the practice version then this would explain why there are so many 'unexploded' rounds around the Lusitania because they were practice rounds they were never designed to explode. Had the RN really fired patterns of 24 Hedgehog rounds at the Lusitania, each containing 34 pounds of Torpex, the wreck would be devistated and very obviously so.
