The sea-surface microlayer (SML), the uppermost millimetre of the ocean, concentrates organic matter and trace metals under intense ultraviolet radiation, creating a biogeochemically active yet physiologically demanding habitat. Whether it harbors a distinct microbial community remains debated, particularly in well-mixed coastal seas where turbulence continuously homogenizes the upper water column. We applied shotgun metagenomics to size-fractionated (free-living, 0.2–3 µm; particle-associated, >3 µm) SML and underlying water (ULW, ~1 m depth) samples collected with and without natural surface slicks in the North Sea near Helgoland (Germany), using a remotely-controlled catamaran sampler. From 44 metagenomes, we recovered 200 high-quality metagenome-assembled genomes spanning 14 phyla, including 59 novel species and 13 novel genera, dominated by Pseudomonadota, Bacteroidota, and Verrucomicrobiota. Differential abundance analysis revealed no significant community differences between the SML and ULW, consistent with intense near-surface turbulent mixing. Instead, particle association emerged as the primary axis of community differentiation: 91 genomes formed a high-confidence cohort of 36 particle-enriched and 55 free-living-enriched members, consistent across both layers. Free-living genomes were markedly smaller (~2.5 versus ~3.8 Mbp) yet encoded higher per-Mbp densities of UV-damage repair enzymes, photolyases, chaperones, flagellar genes, and high-affinity nutrient transporters. Particle-associated genomes were enriched in glycoside hydrolases, carbohydrate-binding modules, and adhesins. Natural slick conditions acted as an ecological filter, depleting most taxa, with Planctomycetota disproportionately reduced. Together, these findings demonstrate that lifestyle strategy, rather than vertical position across a one-metre depth gradient, primarily governs genomic adaptation in the bacterioneuston of turbulent coastal seas.