If you have your heart set on going to the Dolphin tour.....then when you get on ship book it....I know its a "encounter" instead of a swim, but it would at least get you with the animals. Carnival has space, but it goes quick....you will need to book asap when you get on the ship. I just don't know if you can do this on the TV or have to go to the shore excursion desk. (TV I hope)
The swim not just the encounter is available through Carnival on the ship and to answer another question yes you can book the shore excursions through the TV. You can do this as soon as you get on board and I would recommend this as this particular excursion sells out very quickly.