After Phineas had experimented by dunking his tootsies in The Thomas Bath, Thomas gave some more coaching. He encouraged Phinny by demonstrating how not to fear the water & to embrace it:
With such a good teacher, Phineas soon started getting the hang of this bathing business:
And now having more of a dunk:
As you can see, Thomas was guiding Phineas throughout the whole session. I am hoping he told Phineas that bath water is not for pooping in, but I suspect in all the excitement that that crucial piece of information may have been omitted.