I keep my pool blue all year although it is just for the view for 4 or 5 months in the winter. We have solar heaters that adds a couple months to the season and a 330kbtu gas heater but that is too expensive to run that often.
I suppose there are some Yoopers and Canadians who would swim in there all year long but 78f (water temp) is about it for me and my wife wants 84f. The solars will keep it in the high 60s on the coldest days but it might as well have ice on it in our opinion.
I am sure we still get enough sun to keep the water turned over in the winter since you don't really have as much stuff growing in cold water as you do when it is 89.
Winter is a relative thing here. I am talking about 60s in the day and 40s at night on the cold days and that is a month or so.
