This might seem like an odd question since the snake is in a tub, but is there a hide in it? If the snake is that shy, maybe it does not feel that secure in the tub. The first thing I would try is get an appropriate sized hide that fits nice and snug around the snake. Pre kill a rat (cervical dislocation is easy and fast) and leave it in the tub over night. I pre kill the rats for my younger snake and she will wait for almost an hour until she pokes her head out, looks around like she's making sure the coast is clear, and then drags it into whichever hide she is in. It might just be something that simple, but let's go from simple solutions on up.
