Dolphins communicate with body language, distance, depth and telepathy. If they want to touch you they will. No one appreciates being grabbed at and dolphins are no exception. If you want to experience being touched by a dolphin, let go of your expectations and open your heart. Honor your own timing.
Many times there are a few dolphins and many people.
If they've given you quality time and have turned to someone else, honor that and thank them for the time you've had. If you see someone swimming alone in an intimate conversation with dolphins, don't interrupt. It's as rude in water as it is on land. Be polite to other swimmers. Watch out for other people in the water.