It's akin to weak measurement - we can measure which slit the particle goes through without disturbing the interference pattern.
The analogy isn't a problem. The fact that there's nothing small enough to observe photons like we can observe the oil is a problem, but it says nothing about the integrity of the analogy.
In the analogy the water is space-time, meaning "observing" the droplet like we observe photons would require shooting another droplet past or into the first droplet.