Swing an oar around and break someone's nose. After being warned not to swing the Oars around without looking. That would be pretty easy to do.
Walk away from a four you're carrying on your shoulder so only three kids are carrying it and they drop it. That's a good one because its breaking an almost sacred rule of carry your own boat, plus goes against the spirit of teamwork.
Sometimes a coxswain is like, a person with no legs or otherwise physically disabled and small...maybe dropping said small person in the water while setting him in the boat.
Walking a single down to the dock and concussing someone with the stern of it.
Being too hungover to row without puking, at a regatta.
Take up smoking.
I have to say I damaged a boat as a coxswain in 10 th grade while judging the angle to line up to go under an unfamiliar bridge. I did not get in trouble, though nobody was very happy with me. I also caught a crab in a race as a HS rower...not that uncommon with nervous kids. Most stuff that falls under the 'dumb kid mistake' category won't really get you kicked off high school sports. It's kind of already assumed that even well intentioned kids are hormone addled screw ups.
A single incident might just be par for the course with teenagers, and not get you kicked out of rowing, but a
pattern of neglectful screwing up might do it.
Say she gets caught idly swinging an oar, gets warned, swings the oar again (separate occasion) and hits something hard and cracks it. Gets told off, and warned. Somehow manages to have another oar incident. Smacks another rower in the head, ambulance, etc. Maybe knocks the kid into the water? Near drownings are scary.
Or, a couple of oar incidents, then, not watching where the back of a single scull is going and hits someone. Worse, is carrying the single scull, and kind of wiggles it and
jokes about hitting someone, implying that she just doesn't give a damn, is not repentant, etc.
If the idea is that her life is spiraling out of control, this might work better than one incident, where being dumped from the team would look like over-reacting.
It could also show the escalation of her problems.
Also, I liked (well, not really) the too-drunk or hungover to row. Just who you don't want sitting behind you in a skinny, tippy scull: Someone who keeps moaning "I think I'm gonna puke, oh god, I'm dying".
She might get dumped by her team-mates, regardless of what the coach thinks.