Let’s have a moment to talk about John, the cabbie’s victims from ASiP and water.
All the cabbie’s victims in ASiP are John mirrors. Just after we’re shown John saying that nothing ever happens to him and having a gun on hand, indicating that he’s contemplating suicide at this point, we get a string of people who seemingly commit suicide to show what kind of scenarios would push John over the edge.
First, we get Sir Jeffrey Patterson, a man unhappily married to his wife, having an affair with a coworker in a purple shirt. John will be unhappy/suicidal if he remains married to Mary when he secretly loves Sherlock. Sir Jeffrey gets a cab to his death because the ruddy car that was supposed to pick him up went to Waterloo. John will commit suicide if Sherlock does not wait for him, but moves on towards another “water”, another emotional relationship.
Second, we get James Phillimore, a man unwilling to share an umbrella with another man. John will be unhappy/suicidal if he is more worried about what people might think than what he himself wants. James gets a cab to his death because he is literally soaked from the rain and refused to share Gary’s umbrella. John will commit suicide if he does not accept the shelter from emotional turmoil that Sherlock can provide in the form of a steady relationship.
Take care to note that Gary, the one holding the umbrella, is also wet in this scene, so the umbrella isn’t meant to imply emotional detachment but rather shelter from emotional turmoil. All in all, I think it’s actually pretty safe to assume that rain is meant to imply emotional turmoil.
Third, we get Beth Davenport, a woman coping with her responsibilities by drinking heavily. John will be unhappy/suicidal if he does the same. This is the only victim where water is not involved. Take care to note, though, that this is also the only victim where the reason for the “suicide” is not linked to romantic feelings.
Fourth, we get Jennifer Wilson, a woman with a string of lovers who’s been unhappily married for at least ten years. John will be unhappy/suicidal if he stays with Mary and copes by having a string of lovers. But this suicide, the most obvious of the John mirrors (they share initials), also seems to mirror the situation in the period from TBB to ASiB. John is unhappy with his relationship with Sherlock because the tension between them is unresolved and copes by dating a string of more or less indistinguishable girlfriends.
Jennifer Wilson’s coat is wet, but her umbrella is dry. Part of John’s love life is emotional, part is not. The part closest to himself (Sherlock) is the wet/emotional one, the part he uses to ward off emotional turmoil (girlfriends) is the dry/unemotional one. Jennifer Wilson is also wet under her coat collar. She’s turned it up against the wind, but the wind was too strong for her to use her umbrella. John tries to shield himself from his emotions (for Sherlock), but the wind is too strong for him to shield himself with an umbrella/the emotions are too strong for him to ward them off by having a string of girlfriends.
They really thought this through, didn’t they?