The cannons in the Death Knight starting zone, about a third of the way into the chain, are bugged on at least Icecrown. But, I've found a serviceable fix to the problem that anyone can employ.

First off, you CAN still enter the cannons to use them, even though the cursor doesn't change shape. Just walk up beside it and right-click the green-named cannon, and you'll enter.

Second, the cannons are out of place, but they can be 'driven' forwards and backwards in their fire arc. I managed to move a cannon back onto a ship and into position after a bit of practice, so if you run out of targets to shoot, simply try to re-position the cannon or swap to one that's got a better firing arc on the Scarlets.

I don't know what caused the bug, but there is a way to continue through the zone. Hope this works out for people.