You might need to look for a protective overall case that is water resistant.
Hyperion is up front about saying that their back door does not maintain the water seal.
Mugen has 5900 mAh battery and cover supporting Qi charging. They say they are working on waterproof cover which will send to all customers. (It might take them few weeks, but from experience - they do not lie.)
http://mugen.co/mugen-power-5900mah-extended-battery-for-samsung-galaxy-s5-with-battery-door.htmlExtended cover adds QI Wireless Charging functionality to Galaxy S5. It is not waterproof. We are now working on waterproof cover, which will not include QI wireless charging. All people ordering Mugen Power Extended Battery for Galaxy S5 now will get waterproof version of the cover once it is available free of charge!
http://mugen.co/mugen-power-5900mah-extended-battery-for-samsung-galaxy-s5-with-battery-door.htmlExtended cover adds QI Wireless Charging functionality to Galaxy S5. It is not waterproof. We are now working on waterproof cover, which will not include QI wireless charging. All people ordering Mugen Power Extended Battery for Galaxy S5 now will get waterproof version of the cover once it is available free of charge!