Hi! Glad you liked it.
So flying is done through the Bounce Player event, and you are correct that it could be used for swimming! Back when 2.0 first released, I wasn't a fan of the B button being jump, but noticed there was no suitable event to replace jumping on A besides bounce. The issue with Bounce is that it isn't actually a jump - without using variables or timers, which end up being janky, you get Flappy Bird style controls. So I shelved it.
Until I realized you could just make the character a bird and make the flying jumps a mechanic, like how Kirby platforming has flight!
There's no engine ejecting required, but I did mess with the properties (available in Beta 5) to adjust gravity, movement velocity, and fall speed to make for a smoother experience.