They were not in powered suits, but this has some points in common with the 1968 novel Journey of the Oceanauts by Louise Wolfe.
It has three men who walk across the bottom of the ocean. However, instead of suits they have been surgically altered into 'manfish'.
They also go down into a trench and back up. More details can be found in this Kirkus Review. If this is the book you may remember some of the training details in which there is a combination of isolation, dark, etc. to prepare candidates for the journey. As this was in 1968 I suspect that type of scene was cribbed from some of the early astronaut training programs which tried to simulate the isolation of space.
The journey is across the bottom of the Atlantic ocean on foot as young Josh Sparke, his father and ichthyologist Wayne Markham become ""manfish"" a state that is about as believable as the rather watered down characterizations. But this ""inner space"" exploration offer plenty of excitement with mud avalanches, undersea earthquakes and tidal waves, sharks, whales and even an enormous grouper that becomes first a mascot, then a threat. At one point Josh is lost in an immense underground cavern, at another he's buried alive, and still later he's being swept along at over a hundred miles an hour. The pressure isn't just a matter of depth: there's a deadline that they almost miss as the year's supply of air runs out. The somewhat dubious adventures are offset by the number of interesting facts worked in -- ""each codfish lays as many as 9,000,000 eggs at one spawning,"" -- for those who are interested.