I winterized and closed up my in-ground pool last week, here in Cincinnati OH area. Water temp at that time was 65F. I don't have any heater, just a solar blanket used in early part of opening up the pool, and to help extend the season a bit into fall. Usually open up around early may and close up early Oct, give or take how the weather is doing.
My winter cover is a heavy duty mesh that is essentially lightproof. No idea how cold the water gets, but it feels very cold when opening up in spring, like 30's/40's before getting some solar help. I don't think the solar cover actually causes any extra heating, what it mainly benefits is heat loss at night and minimizing evaporative cooling.
Getting into the pool when water temps are 76 is close to my limit. Prefer it in mid to high 80's. 75 and below is just too cold to be worth the shock.