Plankton and micronekton occupy the base and intermediate levels of oceanic food webs and are generally regarded as difficult to quantify. Gelatinous plankton are the most abundant functional group of macroplankton, yet they remain largely unstudied. What little is known of plankton communities has been largely deduced from plankton samplers, optical counters, nets, and towed cameras. We introduce here a survey methodology that used recreational scuba divers to evaluate pelagic community structure observed on popular “blackwater” dives. The most abundant organisms encountered were salps, siphonophores, and ctenophores. Over a 19-month period, environmental data were compared against nightly observed diversity to build a generalized additive model that accounted for 43% of the total observed deviation in biodiversity. The three most important predictors of pelagic diversity were water temperature, bathymetry, and El Niño–Southern Oscillation (ENSO) index.