have you checked for air leaks in the manifolds and pipework? turbo air leaks can reall scream.
a tip to find air leaks in turbo pipework is to spray an aerosol around the joins when the engine is idling, the engine note will change when the aerosol gets sucked in. you can use easystart, but be careful to keep the area well ventilated, and keep it away from the air intake as it will give a false indication!
check all the high pressure pipework from the turbo to the manifold and dont forget the compensator pipes and head to manifold gaskets.
it can help to restrict the air intake a bit with your hand too so there is more suck!
failing that get some to drive the boat with the engine hatch open and use a pipe as a stethescope to source the noise best done on a calm day!. when doing this its best not to get the pipe tangled in the alternator belt!
its unlikely to be the turbos if you have replaced them.
of course it could be something completley different