Most of the beach hotels have lovely swim areas around their piers. You will see a lot of awesome sealife including seahorse, decorator crabs, star fish, reef fish, eels and the list goes on. However the best snorkeling is around the reef. It is fairly calm most of the time. The trick is to be the first one into the sea and the last one back on the boat. It is the bobbing around that makes you sick. Typically traveling to and from snorkel sites doesn't induce nausea.
Drinking ginger ale helps alleviate the discomfort, or any carbonated drink.
Avoid exhaust fumes.
And there are of course patches and anti-nausea meds that can help too.