Yes - Whistle GO is waterproof (rated IPX7)

The Whistle GO is waterproof (rated IPX7). The IPX7 waterproof specification rating means the device is designed to go anywhere with your pet in 1 meter (about 3.3 feet) of static water for up to 30 minutes. The most intrepid pets can take Whistle GO with them through water, mud, rain and snow. We recommend drying the device regularly, especially after swimming, to keep it from rusting. 

Keep in mind, waterproof is not a permanent condition and might decrease as a result of normal wear. Do not attempt to charge a wet Whistle device!