Abstract: The invention relates to a device for monitoring persons in the water, comprising: a carrier apparatus for fastening to a person; a monitoring apparatus held by the carrier apparatus and which has a sensor apparatus and a processor apparatus, wherein the sensor apparatus is connected to the processor apparatus; and a signaling apparatus held by the carrier apparatus and which is connected to the monitoring apparatus, wherein the signaling apparatus has a floatation body and the signaling apparatus is provided for detaching from the carrier apparatus upon a signal of the processor apparatus and rising to the water surface. The invention further relates to a system for monitoring persons in the water, comprising a device for monitoring persons in the water and a base station. The invention further relates to a method and use of the device to protect persons from dangers resulting from a lack of oxygen and drowning.