Whales and dolphins surely do not have ears similar to ours and yet they can pick up ultrasound underwater without fear of water pressure damaging the eardrum. Any idea how the mermaids with ears like us can manage to overcome the water pressure when fully submerged while we need 2 pieces of ear plugs?
Do mermaids have human ears? Or do they have something that is shaped like a human ear with their real ears elsewhere? Note that dolphins use their jawbones to funnel sound to their ears where we use the external portion of our ears for that.
Searching for information about how dolphins manage water pressure in their ears finds http://dolphinworld.users.50megs.com/anatomy.html which says:
A dolphin's middle ear cavity is filled with a highly vascularized (supplied with blood) tissue. When a dolphin dives, this tissue helps adjust pressure on the middle ear.
This would be internal to the ear. Even with external ears for hearing above water, something like this could exist inside mermaid ears. You wouldn't be able to tell without cutting the eardrum.
Note that dolphins lack an external ear not because of water pressure but to allow them to swim more easily. Perhaps mermaids maintain an external ear because their human-looking half spends more time out of water.
I feel the need to point out that humans that swim on the surface sometimes wear earplugs, but nobody who wants to keep their hearing wears them while diving. We can hear perfectly fine underwater, in our normal range, as long as we equalize the pressure. Earplugs are for keeping water out of the ear canal, not for protecting against pressure; in fact, under pressure they become a danger.