Iron-mediated surface-initiated reversible deactivation radical polymerization (Fe0 SI-RDRP) is an appealing approach to produce robust polymer surfaces with low toxicity and biocompatibility, while its application has been limited so far due to the poor activity of iron-based catalysts. Herein, we show that the iron(0)-mediated surface-initiated atom transfer radical polymerization (Fe0 SI-ATRP) could be significantly enhanced by simply using seawater as reaction media. In comparison, there was no polymer brush formation in deionized water. This method could convert a range of monomers to well-defined polymer brushes with unparalleled speed (up to 31.5 nm min-1) and a minor amount of monomer consumption (μL). Moreover, the resultant polymer brush shows chain-end fidelity which could be exemplified by repetitive Fe0 SI-ATRP to obtain tetrablock brushes. Finally, we show the preparation of polymer-brush-gated ion-selective membranes by Fe0 SI-ATRP for osmotic energy conversion, which gives excellent power densities of 5.93 W m-2, outperforming the most reported as well as commercialized benchmark (5 W m-2).