What is a good word for "unable to be usefully applied"?

Solution 1:

impractical/unfeasible/ From The FreeDictionary:

not practical; not workable or not given to practical matters; "refloating the ship proved impractical because of the expense"; "he is intelligent but too impractical for commercial work"; "an impractical solution"

In statistics, it is said that there is no practical significance. A test that has a statistically significant result does not necessarily have a practical significance. For more explanation of the term, you can read here

Solution 2:

I'd say the water advice is "useless" and the practice of drinking water to lose weight I'd call "futile."

Solution 3:

What about the word fatuous? From Dictionary.com:

fat·u·ous  /ˈfætʃuəs/ (adj.)
1. foolish or inane, especially in an unconscious, complacent manner; silly.
2. unreal; illusory.

"Yes, I suppose your advice is technically correct, but it is utterly fatuous to think it important for my diet plan."

Solution 4:

If you're ok with a phrase, things like this are often said to look good on paper:

to seem fine in theory, but not perhaps in practice; to appear to be a good plan. The plan looks good on paper, but it may not work. This looks good on paper. Let's hope it works in the real world.

^{The Free Dictionary}