Kindly, consider the image below:
Can I say for the glass in the middle: "too few water" instead of "not enough water"? Are they equivalent in meaning?
"few" is used with a countable noun: "There are too few apples in the box". This is equivalent to "not enough apples".
"Water" is not countable, so you cannot say "too few water".
You could use "little": There is too little water in the glass". This is equivalent to "not enough"
As others said, "too little" is more appropriate because you use "little" for uncountable nouns. An extra point, though: In my opinion, "too little" is a bit stronger than "not enough". For example, "not enough" could be slightly less than enough (say 70-90% as a ballpark), but "too little" (or "too few" where appropriate) may mean even less than that (the second glass in your image.)