Only copersharks indeed, great white has become rare in Gansbaai

Both places took a whole day, including driving time from and back to Cancun. We spent half a day in Chichen Itza (1-hour guided tour and then self-exploration in the area), then spent the afternoon at the cenote (swimming, lunch, and there were also other attractions in the compound). You can spend as little or as much time as you'd like. There are small restaurants and cafes in the Chiczen Itza grounds, and also an open-air buffet restaurant on site at Chichikan Cenote (very close / walking distance to the actual cenote).