Buoyancy-controlled underwater floats have produced a wealth of in situ observational data from the open ocean. When deployed in large numbers, or ‘swarms’, floats offer a unique capacity to concurrently map 3D fields of critical environmental variables, such as currents, temperatures, and dissolved oxygen. This sensing paradigm is equally relevant in coastal waters, yet remains under-utilized due to economic and technical limitations of existing platforms. To address this gap, we developed a swarm of 25 µFloats that can actuate vertically in the water column by controlling their buoyancy, but are otherwise Lagrangian. Underwater positioning is achieved by acoustic localization using low-bandwidth communication with GPS-equipped surface buoys. The µFloat features a high-volume buoyancy engine that provides a 9% density change, enabling automatic ballasting and vertical control from fresh to salt water (∼ 3% density change) with reserve capacity for external sensors. In this paper, we present design specifications and field benchmarks for buoyancy control and acoustic localization accuracy. Results demonstrate depth-holding accuracy within ±0.2 m of target depth in quiescent flow and ±0.5 m in energetic flows. Underwater localization is accurate to within ±5 m during periods with sufficient connectivity, with degradation in performance resulting from adverse sound speed gradients and unfavorable surface buoy array geometry. Support for auxiliary sensors (<10% float volume) without additional control tuning is also demonstrated. Overall performance is discussed in the context of potential use cases and demonstrated in a first-ever swarm-based three-dimensional survey of tidal currents.