Polar G3 and G5 GPS sensors are not swimming instruments.
Polar G5 sensor
Sensor is splash proof. You can use it in the rain, but not for swimming. To keep the sensor splash proof, make sure the battery lid sealing is clean to allow the lid to close firmly.
Polar G3 sensor
Sensor can be worn in the rain, or when swimming, crossing rivers, canoeing, etc. The GPS reception or signal transmission from the sensor to the training computer will not, however, work in water. To maintain water resistance, make sure the G3 GPS sensor battery cover is firmly tightened. Do not press any buttons under water.