I searched [COCA][1] for `modicum of *`. Here's the list of nouns that had 3 or more results:

> respect, privacy, control, dignity, stability, success, intelligence, justice, effort, security, sense, peace, order, decency, comfort, civility, relief, education, hope, knowledge, interest, discretion, protection, support, talent, trust, truth, understanding, research, decorum, cooperation, credibility, fame, grace, independence, normalcy, respectability, restraint, safety, sanity, wealth, sympathy, time, skill, wisdom, water, self-respect, science, shame, social, light, pride, prosperity, pressure, inquiry, integrity, happiness, courage, courtesy, deterrence, empathy, evidence, exercise, confidence

It's a list of almost entirely abstract items. And when I looked at the hits for "water", I found that all three referred to water as a utility (including one use that used it as an attribute noun: water flow). 

The usage you want would not be common as I couldn't find a similar example even among the lower-frequency results. 

Instead, you can call it a "splash of coffee":

> Where there are no judgements for adding a splash of milk- or just a splash of coffee! — [Cohesive Coffee][2] 

  

  [1]: https://www.english-corpora.org/coca/?c=coca&q=102996952
  [2]: https://www.cohesivecoffee.com/about-us