Do not speak to the dog. Speak to the person instead.
First and foremost, do not distract the dog by petting it, calling to it, meowing or barking at it, or offering it food. The person's health and safety may depend on the dog's ability to concentrate so distracting the dog may result in injury to the person.
You may ASK to pet the dog, but be respectful if the owner says "no." Some service dog owner's will permit petting after they have removed the dog's gear. Others will not. It depends on how the dog was trained, whether the owner has time, and whether the dog is needed to remain on task at the time you ask.
It is natural to be curious about the service dog. It is okay to ask about the dog, but be respectful if the owner appears busy or in a hurry or simply doesn't feel comfortable talking with strangers. Some owners will enjoy talking about their special helper and educating the public about service dogs, but not all will.
It is kind to offer a bowl of water if the dog appears thirsty. However, most service dog owners will not permit their dog to accept treats or food. This is partly due to not wanting to break down the dog's training, and partly because there are, unfortunately, people in this world who will attempt to poison working dogs like service dogs by feeding them poisoned treats.