For dolphin encouters you don't need an excursion at all. Just take a cab to Chankanaab Park and you can do the dolphin encounter there.
Cab will be $6-10 depending on which dock you leave from. Entrance to the park is about $12 or so and I don't know the price of the encounter. When you're finished you can stay at the park and lounge on the beach, complete with waiters who will bring you a pina colada in a pineapple or coconut, rent snorkel gear right there, eat the civiche at Bucanners cafe or walk through the botanical section. There is also a historical museum and lagoon. The best deal around is a trip on your own to Chankanaab. The ride is about 5 minutes from the pier.