The best way to distinguish between these terms is to look at what other terms we contrast them with. 'Still water' is used as a contrast to 'sparkling water'. If it has bubbles, it's sparkling or fizzy. If it doesn't, it's 'still' 'Mineral water' refers to water taken directly from a natural spring or source. This is in contrast to 'tap water', which comes from reservoir. 'Fresh water' is not a term used for a commercial product. Fresh water is drinkable, whereas salt water (from the sea) is not. So, for example, you might say that an island is uninhabitable because it has no fresh water.

"Still water" is "acqua naturale",. "Mineral water" is water from the earth that has a mineral content, intended to be beneficial. ""Fresh water" can mean "the water from a lake or river", that is "not salt water". It also can be used to mean "drinkable water." We would call "frizzante" seltzer water if carbonation were added to still water. There is no good term for bubbly, natural water.

'acqua frizzante (o gassata)' = sparkling mineral water 'acqua naturale' = still mineral water 'acqua dalla spina' = tap water 'fresh water' is too vague so best not to use it or ask the waiter to make it clear what this means if it says this on the menu

Still water and fresh water are similar to acqua naturale for drinking. Mineral water is similar to acqua frizzante. If you are not talking about drinking water, but water in a lake it is a different explanation. This is information about still water: Water that is located in an enclosed lake without a strong current and/or unflowing water. The water is generally not drinkable. Water that has been distilled has been filtered and purified to remove impurities (often from stillwater and saltwater). After a time it will revert to being classified as stillwater due to the effects of longterm storage. Mineral water is water from a mineral spring that contains various minerals, such as salts and sulfur compounds. Mineral water may be effervescent (i.e., "sparkling") due to contained gases.