Disney only offers the Dolphin Encounter. My daughter (9) and I wanted to do the Swim so I booked it today directly with the Nassau office. The Swim is $165 per person (pricey!, but hopefully worth it).
They said that for Grandma to observe the price would be $20.00 (also booked).
They said that I should get off the ship, ask for directions to the water taxi ($3 per person) which would take me across to Paradise Island where the Dolphin Encounter ticket/boat facilities are. We should go to the ticket window, tell them our confirmation #, and board their boat to the island. I assume that this is the procedure in reverse as well. They said it would be about 3 1/2 hours total (I assume there is some education going on first, then changing into the provided wet-suits, then finally going into the water with the dolphins.....just guessing here about the actual sequence!)