Pull the entire manifold off the boat and remove the hose at the riser end...
These are difficult to get apart most times so be reall careful.
Take loose all four nuts on the studs of the riser to manifold and lay the mating surface accross a block of wood with a towel over the wood to prevent scratches.
Put some pressure on the ends of both the manifold and the riser in an attempt to break it loose, you may need to flip it over and over a few times to get it to budge.
If it really does not want to come off, my guess would be the studs are corroded into the riser flange holes, you may need to pull them out all of the way (normally you can only get three out).
You could just make a new gasket, but they are avail. Rex Marine has them in stock (I think Gil Marine still makes them too)
Good luck!
GT