I have a child who has some ASD traits and I'm a swimming teacher who -occasionally, thank God - teaches large groups of 6 year olds in the pool.
Frankly, it's a nightmare - they're waaay too little to really take in the safety issues, they often just think they're there to muck about. They often don't have the listening skills - even NT kids - to work out what's going on in that noisy atmosphere. School swimming is exciting - the coach/walk to the school, the changing rooms, the new teachers, seeing everyone in their swimming costumes etc. And then just half an hour in the pool to actually learn swimming.
One child with SN doing what he/she wants can be enough to disrupt the whole class. You have to try and get behaviour under control somehow for safety's sake.
I have a lot of sympathy for the young teacher.
I also think, as a parent of a possibly-ASD child, frankly if it "worked" and will keep him safe in that context, that's a bit of a good thing. My DS sometimes has to be terrified into not doing stuff that is flat out dangerous.
But you should definitely let him know how distressed your DS was. It may mean they have some trouble getting him through the whole preamble and back into the pool.
You can also make sure your boy has water confidence skills and is good at swimming so he won't mind if someone throws them in the deep end. My 7 and 5 year olds LOVE being thrown in the deep end. Take him to additional lessons if money allows. Classes will be smaller and the swimming teachers will get to know the children better.
Good luck.