How far down the rabbit hole would you like to go?
The iPhone is "waterproofed" with half black tape, half plumber's tape. Its not hard to get a seal that is about the same as what Apple put on it stock. For Samsung, its basically black tape all the way around the front and back. I prefer using the precut adhesive for the S7, so its technically OEM waterproofing.
But I have no way to check to see if it really is waterproof. I can't advertise that it still is and I can't drop a phone in a bowl of water for hours. If I wanted to advertise as waterproof, I would have to buy the $15k nano coating machine and spray every phone. That would be waterproof but is it worth the money?
Instead, I try to make the phone as close to OEM as possible but tell people that its not waterproof. That way its more than likely pretty watertight, the customer will try to not get it wet and won't come back to me upset if it does get wet and dies.