If a session is completed indoors, be sure to check the "indoor session" box. This will override the forecasted conditions with your default indoor conditions, which can be edited in your settings.
Air temperature and humidity are largely irrelevant for swim intensities, so only elevation will have an impact on your pacing. Swim sessions do not need to be marked as indoors.
No, your cardiovascular system naturally adjusts for changes in your environment. Your heart-rate zones will stay the same, but your power or paces will change relative to your heart rate.