A marine engine has an exhaust system comprising a cylinder block comprising first and second banks of cylinders that are disposed along a longitudinal axis and that extend transversely with respect to each other in a V-shape so as to define a valley there between. A catalyst receptacle is disposed in the valley and contains at least one catalyst that treats exhaust gas from the marine engine. An exhaust manifold conveys exhaust gas from the marine engine to the catalyst receptacle. The exhaust manifold has a first port receiving exhaust gas from the first bank of cylinders, a second port receiving exhaust gas from the second bank of cylinders, and a conduit conveying the exhaust gas from the first and second ports to the catalyst receptacle, wherein from the first and second ports to the catalyst receptacle, the conduit only reverses direction once with respect to the longitudinal axis.