Solid wooden bathtub. It's like this, never other than natural occurring death and material utilization... forests are forests for a reason. One wood dresser cared for can last hundreds of years... take care of what you have. If not broke..., or fix it.
This would be the bath and shower head layout at the end of the shower hall. The tub would be stone carved with earthy patterns. The floor would have black river pebbles, and a moss 'bath mat'. The walls would have small foggy windows throughout incorporated into aquatic mosaic.