It’s actually very simple.
Note: I currently don’t have the project right now but my rain setup is very similar
Rain/ Water drops:
Underwater Distortion/Heat:
You can see them in action right here:
Rain:
nAlgg8zg1CA
Heat:
watch?v=08XGwp-xE0o
Those above will be used in my commercial project.
P/S: I tend to avoid using Scene Color because it disables any kind of DoF.