The next decades of space exploration will focus on Ocean Worlds – especially Europa, Enceladus, and Titan – whose liquid oceans beneath kilometers of icy crust are some of the most likely locations beyond Earth to harbor life. NASA is developing numerous ocean-access mission concepts, including the Scientific Exploration Subsurface Access Mechanism for Europa (SESAME) class of drilling robots to penetrate the ice and access these aquatic environments. We propose developing SWIM – Sensing with Independent Micro-swimmers – to dramatically expand the capabilities of SESAME-class ocean-access robotic missions and significantly increase their likelihood of detecting evidence of habitability / biomarkers / life once they reach the ocean-ice interface. SWIM consists of dozens of ~100 cm3 scale, swimming micro-robots (micro-swimmers) equipped with MEMS sensors, propelled by miniature actuators, and wirelessly controlled with ultrasound. The micro-swimmers are deployed individually or as a swarm from a single SESAME robot mothercraft, which has limited mobility once reaching / anchoring at the ocean-ice interface. SWIM enables active sampling of ocean water beyond the reach of the SESAME robot (increasing the chances of detecting biomarkers), as well as temporally- and spatially-distributed measurement of desired ocean properties, habitability metrics, and potential biomarkers (infeasible with a single robot).