Head submersion should not be rushed. However, it can be approached as play or imitation. For example, face dipping for a few seconds as part of a song or game can work. Water safety skills must be part and parcel of swimming instruction. Jumping into the water body, rising, turning, and kicking back to the edge is one such drill.
Most accidents occur near pool or dock sides from slipping in, then, probably, panicking.
Once submersion has become customary, this skill can be provided as part of play, then practiced. Breathing while swimming takes some time to learn, especially for the toddler while in a horizontal position in the water. This can begin when the toddler is exploring the challenges of propulsion by flutter kicking. Another important skill is the back float. One way to initiate it is to have the child learn to rinse their hair by lying on their back in the bath tub.