Cozumel is a great port, and has a number of things to offer. We like renting a small car and driving to the far east side of the island where there are no crowds. The water is rough in some spots, but there are several protected areas where you can swim without an undertow. There are also a few bar-type eateries there as well that serve safe food, drinks, and ice because they have their own filtration systems. We have eaten there almost a dozen times over the years and never gotten sick.
If you want changing rooms and bathroom facilities go to the northern-most end of the coastal road where it turns west to cross back over the island. There is a nice restaurant/bar with facilities there. They try to charge $5 each (we negotiated to $15 for 5 people) if you do not order food or drinks. They keep the beach fairly clean and also have a fresh water pool, hammocks, etc.