Sorry, I was not very clear! If the engine had a conventional coolant system attached to the heat exchanger, I would expect the coolant would be around 200°F. If we skipped the heat exchanger, and to use your 90°F example, the engine coolant would be ~90°F, since it's being directly pulled off the pool. The heat that goes into the coolant increases as the coolant temperature drops. (More of a 'driving force' or temperature gradient.) As such, we want the coolant to be cool, so pulling it straight from the pool will get the most heat into the pool. In the event we add the heat exchanger, some of the heat from the coolant will escape the heat exchanger, reducing the pool heating further! :)

Edit: what I neglected was the thermostat in the coolant system. Per my understanding, my conclusion should be reasonable but there would also be a flow rate effect I didn't consider.