If there's a exhaust leak you should see it exiting through lower crankcase gasket, squirt soapy water on lower crankcase surroundings and check if air bubbles exits gasket while engine is on, try re torquing lower crankcase bolts, could be slight loose.
If the problem doesn't get any better, clean well both mating surfaces and install a new lower base gasket, torque to specs.
Happy Boating
