When exploring the coastline of Yucatan, you will find from rocky cliffs, head-high reed covered areas to lonely sandy beaches everything you can imagine.
If you are looking for sandy beaches, consider going to the north east of the Yucatan peninsula.
Cozumel, for example, has some very beautiful beaches to offer, if you do not mind a lot of tourists.
If you like a more private beach atmosphere, try out the beaches around Celestún on the north west side of the peninsula. The pictures on the left and right will give you a hint of what I mean.

During the time of November, when I was there, it was also nice to walk along the endless beaches in the evening. No sand fleas to bother you, only dogs here and there. Be prepared for little rain showers but most of the time you will enjoy the cool breeze from the ocean.
There are crocodiles in Yucatan and you can creep up on them - but be careful there are no real safety precaution signs. I tried to find them, but wasn´t lucky. Maybe it was the wrong time of the day and they hid themselves at a cooler place. I saw a lot of Iguanas though and big ones.

To try out my camera's new underwater case, I took the chance to book a snorkeling tour in Cozumel. Actually this was my first snorkeling tour ever, as I used to snorkel on my own, and to my regret, I was very disappointed.
The agents in San Miguel will tell you everything to get you onboard and sell their tickets. The tour boats will bring you to snorkeling locations near the town of San Miguel where there is not much underwater life - maybe I have been spoiled by what I have seen and encountered on my scuba dive tours.

I strongly recommend going to El Mirador - located on the south east coast of Cozumel. Find a local guide in one of the beach areas there. The places around El Mirador are much more beautiful than those in San Miguel.
The underwater pictures shown here were shot in both places. You can distinguish the areas by having a closer look to the ground. Around San Miguel the bottom is covered more with sand and less seaweeds, a lot of coral reefs are in bad shape and new colonies are grown on man-made concrete objects.