The mood at the beginning of this story is humorous and sluggish. The narrator states that "It was one of those midsummer Sundays when everyone sits around saying 'I drank too much last night'." The characters are sitting around the swimming pool being lazy and drinking. This is when Neddy Merrill comes up with the idea to "swim" through the neighborhood back to his home. This idea shows the lazy nature of the day where something like this could be seen as an "adventure". He had nothing productive that needed to be accomplished and therefore became "an explorer" finding new routes to his house.
The mood at the beginning of "The Swimmer" is languid and somewhat humorous. The story begins by describing the members of the church and even their priest as having a little too much to drink the night before.
"The pool, fed by an artesian well with a high iron content, was a pale shade of green. It was a fine day. In the west there was a massive stand of cumulus cloud "
The preceding description, with its pale green and soft approaching clouds, in the second paragraph continues this languid feeling within the text.