At the end of Fantastic Beasts and Where to Find Them, Newt helps to obliviate all No-majs by using Swooping evil's venom. Somehow, I don't understand why Jacob only forgets everything he has done together with Newt when he's walking into the rain. There's a guy shown in the shower who seems to lose his memory, too and he's not standing outside.
The rain water contaminates the water in the pipelines and/or overhead storage tanks of the buildings in the city.
As you mentioned, a guy in the shower loses his memory. There is also a shot of a woman drinking from a tap who also loses her memory.
Observational deduction and I don't have any links confirming this.
Also, they could've easily obliviated him instead of making him walk into the rain, but I think that wasn't how it was shown because that would mean that the characters proactively wipe his memory, and I think none of them had the heart to do that. Which is why making him walk into the rain was possibly a kinder thing to do.