You can measure backpressure by tapping off the system near the manifold and using a water tube manometer to observe the backpressure throughout the engine rpm
range. Note that revving in nuetral does not produce as much backpressure as the same rpm when under load because the extra burnt fuel
under load is converted to gas which increases the back pressure. Take the boat
for a run to measure the true back pressure when under load. Compare that to the manufacturer's spec's for maximum acceptable back pressure and decide if any changes are needed. Incidentally when you pull the hose off the exhaust manifold elbow
look inside for carbon build-up, Yanmars are notorious for almost blocking off the exhaust outlet with carbon, adding much more back pressure than a hose restriction.