MedFever – An expanding network of 12 small observatories was established in the Tyrrhenian Sea in 2021 by MedSharks and ENEA, building on the experience of an observation point established in 2016 in the Gulf of Naples.  Very high frequency measurements (15 minutes) at some standard depths (5-15-35m; occasionally at 10-20-25-30-50-60m) were obtained by voluntary diving centers using calibrated HOBO sensors (after calibration accuracy is about ±0.1 °C) in 12 stations evenly covering the Tyrrhenian sea. Due to various technical problems, some records are missing. The dataset proposed here will be upgraded annually.