Found this on eHow. Local laws might differ but I guess you can just send them to the bottom.
Fiberglass boats are an interesting exception to normal fiberglass rules. According United States ocean and environment standards, fiberglass materials cannot be dumped overboard. However, boats made of fiberglass do not fall under these guidelines, and legally they can be left derelict as long as they sink and do not continue to float above the water. These fiberglass boats cannot be used to make man-made jetties or reefs.
Read more: How is Fiberglass Disposed of? | eHow
Some of the old iron mines in the Northern part of the state have filled with water and are popular with divers. One of them has a boat on a trailer sitting on the bottom.