Yes my girls both sleep like babies after their lessons!!
That's a top idea that they have an underwater challenge as that is the hardest part for them to master I think.
My youngest girl is 3, and she is just coming round to loving underwater time. She will now go down and retrieve things from the bottom of the pool with help, which is great. She also LOVES jumping off anything high into the water. So maybe I have one future freediver, and one future cliff diver??
To get her to relax I always talk to her about what you can see under water, such as fish, rocks, shells etc when we go to the sea. And then we go in the pool and I say "ok, I'll come down with you this time". And we go down. Then after 1 or 2 goes we talk and she'll say she wants to try it herself this time. So I tell her the main thing is to not rush, take her time. We just talk about it. And I see her under there, she goes down, hits the bottom with her hand, then calmy rolls over and floats on up, seeing what she can see. She's good because it's a shared interest so she pays attention when we discuss it all.
She has started doing swims underwater across the pool to me as well. I was stunned at this yesterday, it was our first go and distance swimming underwater, rather than going straight down. I showed her and did a breaststroke to her underwater. Then I said "ok you try it." So she did, she dove in, then when she realised she was underwater she adopted a little breaststroke technique, which she'd never been taught. Quite impressive how fast they can pick things up.
I have had her in the ocean before where we've seen fish together, but the next time we go I imagine we'll be able to swim to the bottom and search through rocks, shells etc together for small moments.