Yes - Whistle 3 is waterproof (rated IPX7)

The Whistle 3 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 3 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!