1. A coin sinks in water.
2. A coin sinks in the water.
3. A coin sinks in the water of the glass.
4. A coin sinks in the sea [water].
5. A coin sinks in the river [water].
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[What about my explanation about 'water' and 'the water'?]
In #1 'water' is a generic term. In any water, a coin sinks. In #2, 'the water is the specific water such as #3, #4, and #5. So if 'water is used as a generic term, we need to use 'water.' If not, we need to use 'the water.' I mean 'the specific water'.
1 is fine.
2 - You don't need "the."
3 - It'd be better to say "... in the glass of water."
4 and 5 will be fine without using the word "water."
Yes, using "the" indicates a specific glass or unit or body of water.
Your explanation about "water" and "the water" is generally correct. The usage of "water" or "the water" depends on whether you are referring to water in general or a specific body of water.
In sentence #1, "a coin sinks in water," the word "water" is used as a generic term to refer to any type of water, without specifying a particular body of water. This sentence implies that a coin sinks in any water, such as tap water, rainwater, or any other type of water.
In sentence #2, "a coin sinks in the water," the phrase "the water" refers to a specific body of water. Without any further context, it is not clear which body of water is being referred to. The context would need to be provided elsewhere in the conversation to specify which particular body of water is meant.
In sentence #3, "a coin sinks in the water of the glass," the phrase "the water" specifies that the water being referred to is inside a glass. This sentence implies that a coin sinks in the water contained within a glass.
In sentences #4 and #5, "a coin sinks in the sea [water]" and "a coin sinks in the river [water]," the phrase "the sea" and "the river" specifies the specific bodies of water being referred to. These sentences imply that a coin sinks in the water of the sea or river.
To summarize, when talking about water in general, you can use "water." When referring to a specific body of water, use "the water" followed by the specific name or context that identifies which body of water you are referring to.