When I wash my dad's hair in the shower I have yet to find a satisfactory way to prevent water from entering his ears when rinsing the shampoo from his hair.
There are some creative solutions available for children but never for an adult sized head despite what some may claim.
*Side note: My mother was helping him with his shower previously and
I would often hear him holler about water getting in his ears.
She told me she gives give him two dry wash cloths and tells
him to cover his ears with them while she quickly rinses his
hair. She said somehow water still gets in his ears! During his
next shower I was there helping when he started hollering
about water getting in ears. I glanced over as my mom was
nervously rinsing his hair. She didn't seem to notice my dad
was holding the wash cloths firmly only over his eyes! When
I pointed this out to her she looked at me for a few seconds
realizing this was why water was always getting in his ears
each time! She gently guided my dads hands so that the wash
cloths covered his ears and started to laugh (thank goodness)
and then so could I. When my dad said "What's so funny?" all
we could do was laugh some more.
Oh, and there are those caps that keep water from going in the ear.
Can be found @ Amazon
I’ve seen them at many drug stores.
How do I keep my ears dry in the shower?
Keeping Your Ear Canals Dry. Plug your ears with cotton balls covered in petroleum jelly. Use a clean finger to dab petroleum jelly onto one side of a cotton ball. Gently push it into your ear and, once it's snug, dab more petroleum jelly onto the outside of the cotton ball.
Amazon sells a swim cap with ear pockets specifically intended to keep water OUT of the ears, if dad is particularly worried about this:
https://www.amazon.com/ZmZmBay-Protection-Flexible-Silicone-Waterproof/dp/B099FDJ3W3/ref=sr_1_14?crid=3CUAG7AJOEQV5&keywords=swim%2Bcap%2Bwith%2Bear%2Bpockets&qid=1644269955&sprefix=swimcap%2Bwith%2Bear%2Caps%2C249&sr=8-14&th=1
My mother was petrified of slipping in the shower; so I tried about 15 different things until I had the idea to try water shoes, bought her a pair on Amazon, and she hasn't had an issue since.
Good luck!