Definitely sneakers. You can wear shorts and short sleeves if you wear plenty of sunscreen and bug spray.
If you are going from AC, make sure you wear something that will dry quickly as you may get wet on the boat ride. And a light jacket for the return trip would be useful as it may be near sundown as you cross the water back to AC, and you'll need it if it rains. One of these trips goes to Maruba spa so don't forget a bathing suit too!