At our swimming lessons we use balls and support the LOs in a horizontal position while they kick with their legs and push the ball along with their hands. We also flip them on their back and let them go feet first so they can kick the ball.
Getting them used to having water in their face, ears and heads in general is really helpful. We sing that song about this is the way we wash our face (scoop water over face) this is the way we brush our hair ( water over the head). A watering can to pour over their heads in ever increasing sprinkles is also handy.
Those squirty toys or the heavier Zoggs that sink are good for encouraging then to dive under the water.
As others have said bubble are good too. We first do the mouth in the water, then the mouth and nose and finally the whole face when swimming forwards.
Wheels on the bus with then supported over your knee and using their forearms to make swimming motions in circles or wiper swishes is good for encouraging the right movements.
Most swimming lessons are 30 mins long as that is the optimal time for LO to not lose interest or turn to jelly
