When replacing the exhaust system on your boat, you need to pay special attention to the gasket orientation with regards to the water ports in the riser and elbow.
Your replacement exhaust gaskets will include various combinations of holes and materials.
If your motor is fresh water cooled with raw water cooled exhaust, you will put the metal plate between the manifold and riser. The metal plate will be sandwiched between paper gaskets. This will separate the cooling fluids and keep raw water out of the motor and fresh water out of the exhaust. If your motor is raw water cooled, then you will not use the metal plate at all.
Be sure to use stainless steel studs when you bolt the riser to the manifold. This will help make future replacement easier. On the studs, you want to thread the short threaded section into the exhaust manifold and use the long threaded section to guide and slide the riser into place. You also want to use solid washers on the bolts as they will form a metal O-ring that will add and help in the seal. If you don't use a solid washer or use a split washer, the studs will leak water.
Once you come to the riser/elbow junction, you will be faced with another choice to make. The riser has the exhaust port in the middle and is surrounded by a water jacket with 4 water ports. The elbow has the same setup on its mating face, but careful gasket selection should be taken at this step.
If you install the gasket with three holes in it, water will flow from the riser to the elbow in the lower portion of the junction and the top of the unit will become very hot. This heat buildup will cause the elbow or riser to crack!
Be sure to install the gasket that has one hole with the hole up at the top of the interface. This will force water to fill up in the riser and flow into the elbow from the very top. This will ensure that the entire exhaust system is covered in flowing water to keep the heat levels down and to prevent cracks due to heat buildup.
On the faces of the gaskets, you want to use aviation grade gasket cement to ensure a proper seal on all ports.
Hopefully these tips and tricks will make your exhaust replacement easier and avoid any need to "revisit" the exhaust setup for several years of boating.