I'm trying to figure out if what happens to Neddy is a result of a breakdown, an alcoholic haze, losing everything, or economic change.

In John Cheever's "The Swimmer," I don't know if you are
supposed to find a specific answer, as much as look to his experiences as a metaphor of
his life.

Neddy comes from the upper echelons of society:
he and his friends are wealthy with beautiful homes, servants, and free time to idle by
the pool, party and enjoy their lives.

When he decides to
swim home, I take this as a metaphor for his past: he sees people he knows who welcome
him and are glad to see him. He continues as he stops at each home to feel strong and
self-assured.

However, then the storm hits. This is
symbolic of tragedy striking. It could be any number of things: losing his wealth in the
stock market; running through his money; investing in unwise business deals. The cause
is never clear, but the mood of the story changes dramatically
here.

He visits one family that speaks to his
"misfortunes," things he was not aware of. What is the cause? It could be alcoholism, as
you suggest. There is certainly a great deal of discussion about drinking: the story
begins with everyone saying how they had too much to drink the night before. The fact
that this seems to be a joke at the beginning may speak more to the lifestyle they lead
and less to Neddy's particular problems, but there is no way to know. He certainly feels
the need for a drink more often after the storm, believing that this will strengthen him
and make him feel better. There is enough supporting detail to make an argument for
this.

It could be some kind of mental or emotional
breakdown where he refuses to face the new reality of his life: he has lost everything,
it seems: his house has been put up for sale; he has visited several people, including
his old mistress, to borrow money. The hostess that is so rude talks about him to other
guests in his hearing about his "fall from grace." It could be that he is simply
repressing these details from within a serious depressive
state.

Regardless of the cause, by the end the reader is
aware that the journey Neddy has been on has taken him from the heights of social
acceptance to the brink of ruin, and he seems confused as to what has happened. Either
alcoholism or some kind of breakdown could be responsible. It is up to the reader to
decide which situation is most clearly supported with the author's
details.