The main effect that water has on climate is that extreme occur in cities near large bodies of water because water heats up and cools down very slowly

The missing word is - don't.

The cities that are near a large body of water do not have problems with extreme temperatures because the water, unlike the air, heats up and cools down very slowly. Since the water body is close to the city, than it will significantly influence the climate in it, which means that it will contribute to the temperatures in the warmest period of the year to be lower, and the temperatures in the coldest period of the year to be higher, or with other words, it will result in much more pleasant climate, that will not have any extremes in temperature.