Some embodiments relate to a catalytic assembly for an engine. The catalytic assembly includes a cooling housing that receives a coolant (e.g., seawater). A catalyst housing is disposed within the cooling housing such that the coolant flows between the catalyst housing and the cooling housing. A catalyst substrate is positioned within the catalyst housing such that exhaust gases from the engine flows through the catalyst substrate. The catalyst substrate serves to covert harmful emissions found in the exhaust gases to less harmful emissions. The catalyst housing includes a first fitting and cooling housing includes a second fitting that mates with the first fitting to prevent rotation of the catalyst housing relative to the cooling housing.