|Some lakes, like a shallow private competition site, don't change too much when they cool off. This is because without new fresh water, the impurities stay about the same. Other lakes "die" over the winter season and the water cleans right up, the once-alive micro organisms settling to the bottom. In these deeper, fresh or northern lakes you will see profound changes in water quality.
Overall, the water becomes more viscous, which means thicker or heavier like a higher grade motor oil.
The ski will not turn as a fast, and it takes more time to complete the turn.
Additionally the ski rides with more overall drag and more tip up. The fin slips less too because the fin gets more grip in the stiffer water.
Because the slalom course is a fixed dimension, this can all add up to a significant drop in buoy count.
So what do you adjust ski wise? Well everything. To correct tip attitude, the boots go forward 1/8-1/4". The fin has to go forward with that say 0.100" to keep some side slip happening. The fin area can also be reduced by decreasing depth a little. And the wing - lower angle like 4-6 deg or pull it completely and give the arms a break.
You may never match a summer season score but at least the ski will turn with some handling authority.