In my opinion one of two things happened here. As far as i know there is no functioning cannons at cove/spawn witch means he would have had to have done what crazy said in sailing a ship with a pre built cannon to get around the pvp protection. Another possibility is that if i am not mistaken people have been able to use lingering potions in the past to attack people at cove/spawn witch is just another way to get around/abuse the pvp protection zones. This could be possibly what happened due to “killed by magic” death message applying to potions as well and there is no remains of a ship in cove and if he did sail a ship in someone would have seen him do it witch make me think that he used lingering potions.