There are several reasons that an outboard motor can overheat. Almost all outboard motors are cooled using the water the boat is floating in. This water is pumped up from the lower unit using an impeller and its flow through the cylinder head and water jackets surrounding the cylinder bores is regulated using a thermostat.

There is no water coming out of the telltale

The telltale is the jet of water that comes out from the side of the outboard's powerhead. It is there to tell you whether your water pump is working on not.

The thermostat is stuck closed

The thermostat opens and close to regulate the temperature of the engine to an optimal point. If the thermostat is stuck closed the engine will overheat, if it is stuck open the engine will never get up to temperature and therefore have incomplete combustion.

The water is being displaced by a blown head gasket

In order for the water to cool the engine it must flow in the water jackets that surround the cylinder bores. If the engine has a blown head gasket, the combustion gasses can displace that water causing the engine to overheat.

No two-stroke oil in the fuel

If a two-stroke outboard is run on pure unleaded petrol instead of petrol mixed with oil, the engine will overheat and seize causing it to stall.

The water passages or jackets are blocked with debris

In order for the water to cool the engine it must flow in the water jackets that surround the cylinder bores. If these passages are blocked with sand, mud or salt build up the water will not be able to flow around these passages or effectively conduct heat away from the surrounding metal.