Aside from the alien drowning and the protagonist being blamed, your description matches The Stolen Law, by Anne Mason.
The Stolen Law was published in 1986, and is a sequel to The Dancing Meteorite. Both books are about Kira Warden, a young cadet living on a space station. She is an exo-communications specialist (translates alien languages), and in the Stolen Law she is sent to work on the Vallusian station. The Vallusian's are human-like, but with six-fingers.
There is a description of Vallusian junior officers try to show her how to use a weapon (which requires 6 fingers), and one where Kira is trying to see if she can open an access panel:
...Kira dragged over the nearest empty chair and climbed up on the
seat. From this height she had a better view of the six pads that she
had to touch all at once. After several experiments, she found that
by crossing her hands she as able to make sufficient contact to open
the access panel.
Earlier in the story there is a scene where she saves the life of a junior Vallusian officer from drowning during a training exercise, because she has lived on planets and can swim.
Peering down, Kira saw the swift flow of the water carrying some
debris rapidly downstream. "Don't worry, sir, I can..." She discovered
that Vallusians had no word in their vocabulary for swimming. "I can
manage."
When an officer falls:
Kira stared in frozen horror at the thrashing figure as the current
caught him. Then without any conscious plan, she kicked off her boots
and jumped off the edge of the banks.
The end of the story does have her on trial for breaking a law -- while on "sector status" (on duty as a translator), she is not supposed to reveal anything she learns, and the punishment for breaking that law is death. But when she learns of a plot to kill some other aliens and blame the Vallusians, she takes matters into her own hands. She is eventually caught and put on trial.
One of the commissioners opened the hearing, then read the list of
charges. Kira could not bear the sight of so many familiar faces. She
looked down, listening to the charges against her. She had expected to
be accused of violating sector status, but that was only the
beginning. Thefts, lies, deceptions, endangering, impersonations --
she wasn't even sure what all the charges meant. There was only one
charge that mattered, the one they would execute her for.
Kira's age is never mentioned, but somewhere between 15 and 18 sounds about right. There is a bit of romantic undertones with one of the other human cadets:
Kira turned to wait for him, feeling a warm glow that had nothing to
do with the sun shining down on her.
This is one of my favorite YA books from the 80's, so it's definitely worth reading again if you can find it.
