All Fi Collars are waterproof.
The Fi Series 3+, 3, and 2 Collars were tested to the IP68 standard under water. This means your dog can swim, play, dive in any form of water, salt water, fresh water, chlorine, as Series 3+, 3, and 2 Collars can be submerged in water up to 1.5m deep, up to 30 minutes.
Additionally, the Series 3+ and 3 Collar are tested to the IP66K standard, meaning a water jet can spray the Series 3+ or Series 3 Collar at 150PSI in 1/4" stream up to 3 minutes without causing damage.
For tips on how to clean your Fi Collar after a swim, see: Cleaning the Fi Collar.