My main aim is to try and find the best word to use when describing a freshwater lake.
But what if any is the difference between the following: 清水, 淡水 and 自来水
清水 clear water; 淡水 fresh water (contrast to salt water); 自来水 Tap water, 30 years ago most of the chinese people had to fetch water from nearby rivers, lakes or wells by using buckets (挑水). Tap water goes through pipes to homes, people think these water go to their home automatically (without 挑水), they created the word 自来水
I would recommend the word 淡水.
清 means “clear; distinct; complete; pure”. However, there is slightly difference of shade in the word "
清水"，清 emphasizes on "clean",whose equivalent is "clean water" [Antonym:脏水dirty water] but not as same as "clear water" in English.
As for 淡水Freshwater，自来水[lit. SELF-COMING WATER]Tap water, Fivesheep has given a good answer above.