When you move into a new apartment, in America at least, it's normal to give the landlord an amount of money that they hold in case you damage the property. This security deposit is given back to you when you move out. So the joke is that Sensei will never get her money back from the landlord because she's ruined her bathtub. She can kiss that security deposit goodbye.
If the tub was now clean, and Komaro and Sensei are bathing in it, why would Sensei lose her security deposit? The joke only works if Sensei can't use her own bathtub because it's full of shit, and must use Komaro's bathtub instead.