Seven Mile beach is beautiful. As Penny noted, not great for snorkeling though as there are not any fish attractors. You could split the difference and spend a few hours there, and then go to Abanks'. There are plenty of fish there, and a small man made sandy area, not exactly a beach as it separated from the water by rocks. They do have rental chairs. This is very close to the tender dock. They have great food and an awesome view of the ships.
is a link to a location map.