I also went in March and wore a 3mm. I was comfortable. The island is the most southern of the Bahama Islands. If you never been there, plan one day to do the tour with the mayor. Most diving sites were very good I liked one called telephone pole. The site is self explanatory when your there.
Stay off the sand as much as possible, noseeum's are really bad and you will get bit up.

I used avon skin so soft and it worked o.k. to keep them away. Listen to the islanders and they will tell you how to keep them away.
Are you using Riding Rock or Club Med? Club Med boats are mostly cattle call boats, 50 or more. If you using Riding Rock its much smaller, biggest boat will carry 20.
Have fun on your trip it a nice quite little island.