I've seen some people have this bug happen accidentally, but I've seen this done with consistency with my method.
As the Mage is being called out to freeze your ship's guns, have a tank in mid-air taunt them as they are running out but before channeling. This is made much easier with a DK's Death Grip, which has double the range of most tank taunts. The Mage will run around on the other faction's boat without channeling and not attacking anyone, your guns won't get frozen, and no more mages will spawn completely trivializing the fight.
While the fight is already easy, this makes "On a boat" a cakewalk, and likely will make hardmode a complete breeze.
So Far I've seen our guild's DK tank perform this on 2 10mans and 1 25. Let me know if you've any questions!