I swim at a local gym. The pool has 3 lanes, each with a clear stripe tiled down the center. If more than 3 people are using the pool at a time, the standard practice is for 2 people to double up in a lane. The newcomer generally asks the person already in the lane if they would share, and both swimmers/exercises stay on their side of that lane’s center line. Never been a problem.
Yesterday I get there, and there are 2 people in each of 2 of the lanes, and one lane with a only single guy swimming.  I stand at the end of that lane waiting for him to get there, ask him if he minds sharing, and set off.  I always make a special effort to stay on my side of the lane.  Yesterday, I often brushed up against the wall of the pool.  About 10 laps in, he kicked me as we passed.  I thought “No big deal.”  Next length he pretty much ran right into me, bringing us both to a stop.
I said, "Excuse me, please stay on your side of the lane."
When he did not respond, I said, "You were swimming down the middle.  See - you are standing astride the center line right now."
He said, “It was hard to see where  I was going, because I was doing the backstroke.”
I responded, "Perhaps you ought not do the backstroke when you are sharing a lane."
End of story.  We both swam off.  Eventually he moved into another lane when the people using it left.
I thought I showed pretty excellent control, and expressed myself appropriately in the moment. My wife thought my final statement was - I don’t know - bossy? Critical? Instead, she thinks instead I ought to have simply said, “Please don’t run into me.”
I can imagine this sort of thing recurring in this relatively well-used public pool. So, was there ANYTHING wrong with how I handled it? Any other way you would phrase it?
 she told me the practice was for folk to each take 1/2.  Since then, that has been the case 100% of the time.  The lanes are wide enough that there is no problem sharing doing the crawl.  When I have a lane to myself, I mix up breast and back, but when I share, I just crawl.
 she told me the practice was for folk to each take 1/2.  Since then, that has been the case 100% of the time.  The lanes are wide enough that there is no problem sharing doing the crawl.  When I have a lane to myself, I mix up breast and back, but when I share, I just crawl. Seriously, the place seems pretty clueless about what goes on in the pool.  But for the most part, it seems to work out just fine.  The vast majority of folk in the gym seem to do things other than swim.
  Seriously, the place seems pretty clueless about what goes on in the pool.  But for the most part, it seems to work out just fine.  The vast majority of folk in the gym seem to do things other than swim.