These can cause great pain, multiple sleepless nights for the parents and even hearing damage for the child.
We have evidence indicating that bathing a child, particularly one who wears diapers, can introduce the bacteria from "not completely clean before bathing" bottoms, into the ear, as the ear either gets under the water or bath water splashes into them.
Try to either keep the head well above the bath water and wash the head with a separate bowl of water or just use a shower with them lying in the bath.
The evidence was provided by someone close to us who had a handicapped daughter who always had to wear diapers, had multiple ear infections and heavy doses of antibiotics and visits to the hospital and these continued until the bathing methods were changed as mentioned. Then there were no more ear infections.
Wishing you a Happy New Year