I have had these apart before. I can't remember exactly without an engine in front of me. So appologies if I confuse things.
There is a exhaust muffler/manifold inside the drive shaft housing which sits in there tight with rubber dampers. This can be a nightmare to remove. You need to remove the driveshaft housing from the engine mounts, and then prize the two parts apart along the gasket pictured, pulling the manifold (held tight by the rubber dampers) out the driveshaft housing. The manifold is bolted upwards, so there is no easy way around this.
Have a look at this diagram; 1992 30ELRQ Yamaha Outboard UPPER CASING Diagram and Parts