How can I fix floor cracks in bathtub (enamel?) coating?
Most home centers will carry a porcelain touch-up paint. I have used it for chips but not for repairs as large as your photos show. Trying it won't make it any worse. As an alternative you can see if you it is possible to identify the tub manufacturer. It is possible that it still is under warranty. Several full scale repairs options are available with out replacement of the tub. One type involves installation of a specially molded liner that covers the entire tub. The other involves coating the tub with an epoxy like material. Neither of these are do-it yourself projects but generally are less costly than having the tub replaced. A local search for Tub-refitters should yield some results.