The one thing that has bugged me is why indoor swimming pools cannot be used in Winter. They are indoors and surely shouldn't be effected by the winter cold, ice and snow etc. Whenever my Sim goes to the Harbor Quarter Gym in Winter the pool is unusable for Powerhouse group meetings. I can understand that pools outside freeze over but not indoor ones.
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Geothermal
Natural heating lowers utility bills by generating power, and keeps any pools warm.
I built a pool in the basement of the library in the Uni town so my mermaids would always have somewhere to swim. They haven't had any issues freezing or being cold when using it in winter. I have not checked since the patch or latest update though.
Other than "Swim laps", Can't use an indoor pool for swimming. Weird thing is, can swim laps in an outdoor pool during a blizzard, heh heh!
Seem like it shouldn't be too difficult to fix. Like growing plants "indoors" all you need it a roof. Even hovering way up a few levels. Also counts as "indoors" for Sims.
P.S. Apparently you can use the toys too even in an outdoor pool in winter like the diving board.
I have multiple indoor pools that sims use in winter. Among those that I have added include: an indoor pool in the Fontainebleau Penthouse in the Arts Quarter of San Myshuno that has glass walls to see the city; an indoor pool in a massive gym that I reconstructed in Oasis Springs where I let sims from different worlds visit in winter for swimming; the gym in Del Sol Valley has a pre-made indoor pool. It's not quite the subject but I added an outdoor pool to the gym in Willow Creek and various outdoor pools in some of the worlds.
I have a large greenhouse with plants growing year round just fine. The only time I've seen them dormant is if I moved them inside from outside during the winter, but after a few days they woke up.