AF-S means different things in different places on a camera...
When setting the type of autofocus, AF-S means single AF (you press the shutter button halfway, and the lens autofocuses) as comapred to AF-C (the lens is always autofocusing).
When looking at lenses, AF-S means that the lens has an ultrasonic motor inside of it that moves lens elements to autofocus. Some other lenses are AF-D, and a little motor inside the camera body attaches to the lens to autofocus it, through something that looks like a tiny screwdriver.
The problem with the Nikon D3100 is that it will ONLY work with AF-S lenses, and won't autofocus AF-D lenses. Two of the most popular underwater lenses for Nikon cameras are the Nikon 10.5mm and the Tokina 10-17 FE, and neither of those lenses will autofocus on the D3100. Another popular underwater lens is the Nikkor 60mm Micro, which has two versions: a cheap AF-D version and an expensive AF-S version. The difference in price between the two 60mm lenses often makes up for buying a more expensive body (D90, D7000, D300) that can work with the cheaper AF-D lenses.
