Are fish averse to extreme heat?
Temperature affect pretty much all metabolic pathways. There are incentive in fishes as well to control their internal temperature.

Control of Body temperature can be categorize along two axes, one concerning the source of heat and the other the magnitude of temperature variation. Please have a look at this post to understand the terms endo-, exo-, poikilo-, homeo- terms. 

Below is a figure to give a general overview of the diversity of species in terms of temperature regulation.



As you can see fishes are found in different parts of the figure.

So, yes there is incentive in fishes to control their internal temperature and to avoid extreme heat as well as extreme cold.