Marine microbes are fundamental drivers of marine biogeochemistry over global Ocean scales. Despite the large-scale influence of their activities, many of the key biogeochemical processes that marine microbes mediate occur within specific microenvironments or chemical hotspots that occupy only a tiny fraction of a single drop of seawater. The scales of chemical heterogeneity associated with these features are compatible with the movement of individual cells, so behaviours including motility and chemotaxis may have considerable importance even within the ostensibly sparse expanses of the pelagic ocean. Traditional oceanographic tools and approaches prohibit examination of these microscale behaviours, so we have applied a suite of microfluidic tools to measure behavioural responses to simulated microscale chemical seascapes. Using both laboratory-based, and more recently, in situ microfluidic experimental platforms we have found that planktonic marine microbes have the capacity to rapidly sense and exploit microscale chemical patches and gradients, within time-frames consistent with the expected persistence of these features in the environment. By coupling microfluidic platforms with ecogenomic approaches we have determined the composition and functional capacity of the microbial assemblages responding to specific chemical cues in the environment. These laboratory and field-based experiments have revealed that bacterial motility and chemotaxis are not only important phenotypes within spatially confined microenvironments like biofilms, but are widely employed by planktonic microbes inhabiting the pelagic ocean. We argue that the propensity for planktonic marine microbes to “swim in the sea” will have widespread implications for ocean ecology and biogeochemistry.