What is one reason lakes, rivers, and other water resources should be kept free of pollution?
Humans use and rely on these water resources, and restoring them or filtering them so that the water can be fit for humans again is costly.
There are obviously many reasons to keep our waters clean. One is that we drink water, we bathe in this water, we brush our teeth with it, swim in it, fish in it, etc. Cleaning and restoring a polluted water source is costly and time-consuming.
The economic effects of pollution potentially include restoration costs, lower property values in surrounding areas, loss of income from tourism, decline of fishing industry, and so forth.
For example, an industrial chemical spill in 2005 destroyed the Songhuajiang River in China. The economic costs of this pollution have been estimated at 2263 billion CNY (source).
80 million dollars was spent on attempts to restore the Sheboygan River in Wisconsin from 2012-2013.
Even if we don't rely directly on a lake or river, we are connected in some way or another to ecosystem services the water contributes to. There are many economic impacts of water pollution and many ecological effects as well which have cascading effects.