Steelhead most definately do have to acclimate, both when leaving their natal stream/river as smolts to the ocean and when returning to their natal stream/river. Not doing so would be deleterious(deadly). It is well understood in Fisheries Biology that physiological changes must take place in order to adapt to differences in salt concentrations of both freshwater and marine enviorments. It takes some time, around a few days to weeks for this acclimation to take place. The mouth of a river (estuary) is a mixture of freshwater and saltwater and is a place for acclimation. In the ocean, teleosts (bony fish), such as salmon, which includes steelhead, drink copious amounts of water and urinate minimally while freshwater teleosts drink very little and urinate profusely. This is done to balance the concentrations of salt in the water versus the salt in the fishes body and is part of osmoregulation (regulating the effects of osmosis). NaCl, sodium chloride, is very abundant in the ocean and much less so in freshwater. So acclimatiom is crucial to the survival of the fish.
The fish seen rapidly ascending a river have already acclimated to freshwater. These fish have lingered long enough in the estuary to tolerate the freshwater. The link below explains how this change occurs.
Time course of the salmon's acclimation responses
The behavioral (drinking or not drinking) and physiological changes a salmon must make when moving from fresh water to salt water — and vice versa — are essential, but cannot be accomplished immediately. Thus, when a young salmon on its seaward journey first reaches the saline water at the mouth of its home stream, it remains there for a period of several days to weeks, gradually moving into saltier water as it acclimates. During this time, it begins drinking the water it's swimming in, its kidneys start producing a concentrated, low-volume urine, and the NaCl pumps in its gills literally reverse the direction that they move NaCl (so that they're now pumping NaCl out of the blood and into the surrounding water.
Likewise, when an adult salmon is ready to spawn and reaches the mouth of its home stream, it once again remains in the brackish ( = less concentrated than full-strength sea water) water zone of the stream's mouth until it is able to reverse the changes it made as a juvenile invading the ocean for the first time.
Taken from "Acclimation of Osmoregulatory Function in Salmon"