Oil contamination is the most common pollution that is seriously harmful to ecological environments and human health. There is a huge demand to develop an effective and facile strategy for the separation of oily water. Here we report a universal self-assembly coating method via a one-step dopamine (DA) @gamma-aminopropyl triethoxysilane (gamma-APTS) Schiff base reaction for highly efficient separation of oil-water emulsions. The addition of gamma-APTS not only accelerates the DA polymerization but also reduces the aggregation of polydopamine (PDA). Interestingly, the newly designed composite membrane possesses both high hydrophilicity and underwater superhydrophobicity. Under gravity-driven separation conditions, the membrane displays superior efficiency to separate oil-water emulsions with an ultrahigh water flux and splendid anti-oil-fouling ability. The coating layer also exhibits multiform robustness and stability owing to the formation of Schiff base covalent bonding. Importantly, the coated membrane has the ability to be used in a dry state without sacrificing its permeability ability. The desirable stability, simplicity of fabrication, and versatility of the DA@gamma-APTS coating may open up new insights to molecularly design next-generation membranes with superior hydrophilicity and underwater superhydrophobic for oily water separation and purification.