G. A. SALA, TO SIR AUGUSTUS HARRIS, ON PASSING THE PALACE THEATRE:—“I SAY, GUS, THINGS LOOK A LITTLE LIVELIER HERE THAN WHEN YOU AND I WERE IN THE SWIM!” — from The Entr’acte and Limelight (London) of 10^th March 1894
(Augustus Harris (1825-73) was a British actor and theatre manager. George Augustus Sala (1828-95), was an English journalist and author.)

The noun swim denotes a part of a river or other piece of water much frequented by fish, or in which an angler fishes. For example, in An Encyclopædia of Rural Sports (1840), the English veterinary surgeon Delabere Pritchett Blaine (1768-1845) explained:

Float angling in the Thames for barbel is also a favourite practice with some excellent fishers, who by this means have a considerable advantage over that afforded by the leger-line, or stationary bait method; for with the floated bait it is very common to take roach and dace as well as barbel, which greatly varies the practice, and increases the sport; but it is a method principally applied to the more quiet swims.

Figuratively therefore, the word came to mean the current of affairs or events, especially the popular current in business, fashion or opinion, in the phrase in, or out of, the swim. Macmillan’s Magazine of October 1869 published an article titled Oxford Slang, which contains the following:

A man is said to be “in the swim” when any piece of good fortune has happened, or seems likely to happen, to him. To have rowed one’s College-boat to the head of the river,—to have received a legacy,—to have made a good book on the Derby,—are any of them sufficient to have put one “in the swim.” The metaphor is piscatorial, “swim” being the term applied by Thames fishermen to those sections of the river which are especially frequented by fish. The angler who casts his bait into these may depend upon sport, whereas his neighbour at a little distance may not have a nibble, being “out of the swim.”