With lots of experience in NYC, Get Real Surfaces is the place to go for interior concrete finishes. They do thin concrete wall finishes or monolithic concrete tubs. Check out their standard products, which include tubs and sinks, or send them your custom design.
Their standard tub (not the one pictured up top) is a concrete take on the Spoon Tub by Agape. Peruse their gallery of NYC projects for more great concrete finishes.