The North side of Cozumel is a wonderful area to stay if you're looking for a slow-paced area of the island, great snorkeling, and proximity to downtown San Miguel.
Along this side of the island, which is just 10-15 from the airport and 5-10 minutes from town, you'll find a mix of hotels, condo buildings, and some residential areas. This side of the island is where the majority of the prominent individuals also live (owners of large businesses, politicians, etc) so it's also a very safe area of the island as there's lots of private security, along with a military airport/base right at the start of the road that heads north from town.
Along the entire road north, which stretches a few miles, there's also a nice walking/bicycle path, so you can safely walk or bike to town. Halfway to the end of the road, there's also a marina. This marina, unlike the two marinas south of town, is largely used by private boats. There are a few fishing and diving charters that do go out of the marina, but they all head south, and, if you have a trip planned, more than likely you'll not be meeting your boat at this marina.
If you're a big snorkeler this is a great area to stay. The shoreline has a big drop off and there are coral formations and reefs right along the shore. So you can just jump in the water and snorkel to your heart's content. Often, there is also a nice gentle current, so it's entirely possible for you to jump in one spot, snorkel with the current, hardly having to kick to propel yourself, and get out and walk back to your condo building or house.