Could it be the plug hole? Can you maybe get one of those fancy plugs with a floating duck/fish etc...? Make it more fun? I know the noise of the plug used to freak out ds1 when he was around that age, then my fabulous (NOT) bil told him to watch out or he'd get sucked down the plug hole!!!! It took a while and we ended up going into the bathroom at a different time of day and getting him to stand in the bath, over the plughole to show him he just wouldn't fit and that it was safe!

Does she wear goggles in the pool? Can she have goggles in the bath and some things that sink so she can make a game of looking down into the water with them? We also ditched the bath toys idea and ds1 chose a couple of his plastic toys to take into the bath to play with. He's now 5 and still loves his bath with batman, an old mini m&m's container, a plastic label from a container (this is currently a treasure map), and an old bubble bath bottle as a squirter.

Good luck! If it comes to it and you can't get her in I like mum2romones suggestion and bath her in a paddling pool!