You didn't say if you mounted directly to an aluminum boat with an isolation material between the charger and hull.
By the way, their statement makes no sense. If you mount it directly to an aluminum boat, then there is no isolation material. The word "directly" implies directly contact, and if there IS an isolation material, well, then it's not "mounted directly".
Maybe what they intended, or should have said was: "Do not mount in an aluminum boat with an isolation material between the charger and hull."? I'm guessing they want the charger's case electrically connected to the hull, which would mean metal to metal. Call MK and ask this outright.