how to use a photodiode to count water drop
You are looking for a logic level on an analog property with many variables that could span several decades without careful design.

Current \$I_f\$ on emitter, PD gain resistor (0.5mA/mW *R=V) or similar into TIA amp with gain.

The biggest variable is optical scatter around the water drop, thru the tube  and thru the drop. Then alignment of a tiny aperture to restrict the scattering to &lt;&lt;1 deg or just enough to capture the drop and PD detector.

Design details


Recessed Emitter and PD detector are both critical to reduce scattering.
Using a TIA, or a slow 1~10M Rpullup to a buffer or comparator to reverse biased PD to gnd. ( assuming slow drops)
-start taking measurements for distance then path loss with fixtureat reduced distance
align for maximum signal variation and use a comparator with xx% hysteresis or a Schmitt trigger gate with 33% Hysteresis if you get enough swing with gain and offset tweaked with pots.
With testing and trial and error using a black heat shrink tube extended over each 5mm part you can make a 1cm long aperture 5mm diameter that may shrink down to 2mm (est.) which is about the size of the chip inside.
Layout is critical for stable repeatable logic levels with good margins. i.e. 10 to 90% of Vcc.
Clean Vcc and twisted pairs with CM choke for noise immunity is important.