[mykwyner]

"I'll bath the baby" is ungrammatical because the sentence has no verb. This, of course, does not preclude a large number of native English speakers from saying it that way. (Although I've never heard that usage)

Many times, nouns are pressed into service as verbs because the exact verb did not exist before the noun was invented, or because it is convenient to do so. When we call someone on the phone, we phone them; when we add oil to a machine, we oil it; when we use text messaging, we text someone. Since bathe already exists as the verb form of the noun bath, then we can not use bath as a verb.