Booking cruise excursions through carnival or waiting till port arrival to pay less?

my husband and i are taking our honeymoon cruise this coming week. the only excursion we are interested in is swimming with the dolphins in puerto vallarta through carnival this costs $140.00 i heard that if you wait till you get to the port and just have a cab take you to the dolphin center it is a lot cheaper but when i look online the prices are similar and i am also worried that if we wait we will not get a space does anyone have any input on what we should do?