Let me add to Karries answers.
For Kayaking there are a couple of places. There is one on 80 that I can not remember the name. The other is north beach Kayak at Lazarrotta creek. Both are good and they rent kayaks and do guided trips. Do a search for the phone numbers. The back river / little Tybee area is good for kayak tours.
2. No there is no snorkeling. Due to the water color you want be able to see anything if you did go.
3. DOGS ARE NOT ALLOWED ON THE BEACH. Not yelling I just want to be clear on this. you may see people with a dog on the beach but the entire beach both front and back is no dogs. They will give you a ticket and it will be from 100 to 300 plus dollars. I don't like that but I understand the reasons but which they would allow it at given times and places.
4. The bugs will not be a problem most of the time on the beach. Inland or near the marshes they will be a problem. Make plans to have bug spray when you go kayaking. Mosquitoes and sand gnats are the big problem. Sometimes you will have a horse fly or deer fly after you.