(In the). In society. The upper crust of society. An angler's phrase. A lot of fish gathered together is called a swim, and when an angler can pitch his hook in such a place he is said to be “in a good swim.” To know persons in the swim is to know society folk, who always congregate together.
“Cottontree, who knows nearly everybody in the swim of European society ... informs him that Lucy Annerley is the daughter of Sir Jonas Stevens.” —A.C. Gunter: Mr. Potter of Texas, book iii. chap. xiv.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Swim from Fact Monster: