Well to start off, hot prisoner day just makes REDs redder. Nothing else as far as I'm aware. Generally isn't picked but I think it shouldn't even be an option
Underwater day just isn't fun, all movement is limited to swimming, the screen becomes blurry and shaky and it also breaks lots of the games e.g. climb etc. Also pyros/demoknights just have a really hard time on either team since flame based weapons don't work and demoknights on BLU can't charge.