Flowergirl, I'm not a swimming teacher, so I'm no pro. But I don't like the dunking thing.
My LB started swimming lessons at 15 months. Because I was heavily pregnant, his father took him. He was the only kid in the lesson, lessons where 20 mins long and the mommy (or in our case daddy) has to swim with the kids.
The first thing the teacher did was to chat with Stefan outside the pool and get him used to her. Then she asked him and my DH to sit on the step and kick with their legs in the pool, she sat on one side of Stefan and DH on the other side. After the kicking song, she showed him how to climb in safely (the way you teach a crawling kid to climb down the stairs)
She then showed him that he could walk and stand on the step, so that he knows he is safe there. They did a lot of other activities that goes along with baby songs and the whole time my DH held Stefan while she stood in front of them and instructed them about what to do.
The songs include one or two where they blow bubbles and others where they lie on their backs etc. With one song she lets the kid hold on to the side and shuffle with their hands all the way to the step where they climb out and back in again. She does this so that they would have a way of getting to the step in any pool.
Her focus is more on water safety than on learning to swim.
When Stefan was 18 months old, I started taking him to lessons in the mornings and we joined a group of two other boys with their mommies. He started swimming without me in the water after the winter break this year, he was just over 2 years old.
He now swims 5 meters (just got a medal at the gala for 5m swim, proud mommy!!!

) and I believe he is water safe. Not that I would let him near an open pool without me right next to him. But I do think if he had to fall in, he would be able to get to the side and shuffle to the step.
We also did one lesson a term with normal clothes and nappies on, where the kids had to do everything that they do in a normal lesson with clothes on. She would then also let them get into the deep, cold, big pool where the club swimmers train. She took them about a meter into the pool, with clothes on, and they had to get to the side and then climb out.
Stefan really loves her approach and he is swimming like a star. He loves swimming and the water and I have never seen any kid at the pool who didn't want to do something after some nice relaxed loving encouragement. She also won't force a kid out of his safe space.
Its all play-play, in the lesson, but they learn sooooo much.
I started my LG in September this year at 9 months, and she can already blow bubbles, get out from the side of the pool and shuffle to the step to get out (she's 1 now). She has been underwater a few times but it was fun and she loved it, it was not a harsh dunking.
This is just my experience and I must say that I love it. There is many ways to teach a child the same thing. And maybe our way takes a bit longer (although I don't really think so) but the result is water safety and a love for swimming. So I'm happy!