Water temperature is a critical water quality parameter that influences physical, chemical, and biological processes in surface water bodies and aquifers; however, the patterns and drivers of the thermal regimes of coastal waters remain poorly understood. Given the pronounced spatial and temporal variability of coastal water temperatures, conventional temperature loggers that are limited to point-in-space measurements likely miss complex thermal patterns. Satellite-based remote sensing reveals large-scale temperature patterns in the ocean and potentially large estuaries or gulfs, but this technology cannot capture the small-scale thermal heterogeneity that is relevant for ecosystems.We combined conventional temperature loggers with UAV-based thermal imaging and distributed temperature sensing along a fiber-optic cable to yield insight into coastal thermal patterns. We present summer water temperature data during a heat wave for a coastal lagoon located in Prince Edward Island. Results highlight discrete thermal anomalies from direct (intertidal springs) and indirect (coastal baseflow) groundwater discharge points. Thermal imaging was able to capture spatiotemporal variability of intertidal springs due to density differences from salinity variations resulting in fresh, cool groundwater floating to the surface of the water column; but these thermal plumes were more obscured at high tide. Distributed temperature sensing at the bed revealed saline and freshwater water that was not apparent at the water surface. Results enable us to better understand hydrodynamic processes that drive complex spatiotemporal thermal patterns. Such findings are critical to help us understand coastal mixing processes, present-day estuarine thermal regimes and associated habitat distribution, and thermal sensitivity to future climate change.