Can be that she’s afraid to pee or poo in the bath
or confused that a swimming nappy goes on in the pool.
Explain that bath time is different from swimming in the pool.
Show example from siblings or yourself.
Instead of a swim nappy you could use reusable Containment Swim wear (to prevent confusion)

kids do this! it can just be a phase or maybe even as the other mums have suggested.

Just let her know that bath time is different to swimming and if she still insists put her in the shower for a quick wash. It should be different enough for her to be ok.

Sounds like she is ready to potty train if she fears an accident happening in the bath. A shower with Mummy may help her understand the need to wash.

Time to mix it up a little. Pop her in the shower now and don’t make a big deal about it.
Kids go through phases. A quick shower and dressed and on with your day.

We put a potty in the bathroom and made it fun to go before the bath well before my son actually toilet trained. Might be worth a try?

Had a problem with one child he loved his training pants on when bathing. He had had an accident in the bath and knew it would contain it. No he was not in training pant (cloth ones) he would demand his brother’s one. I got him into the shower with his father so show he could go without pants.his body parts were normal and accidents do happen.
Just let your daughter know swimming is different to bathing where everything needs washing.

Why indeed would she want a nappy in the bath?! It would have to be so uncomfortable once it got wet. You may just have to insist on no nappy, put your foot down, you wouldn’t be able to wash her properly with a nappy on either

She might becomming aware of her body and its functions. My daughter got really funny after she accidently did a poo in the bath and was scared it would happen again,