in Wisconsin, a snowmobile racing over lakes or rivers is legally a boat. Problem is, that means it has to be slower than the speed limit, and sink

Snowmobiles modified in any way to aid in over the water transportation are considered boats and must abide by all boating regulations. This would include the prohibition from operating at faster than slow-no-wake speed with 100 feet of shore. Water skipping events are exempt from these restrictions; however a local Water Exhibition permit is required.

http://dnr.wi.gov/files/pdf/pubs/le/le0201.pdf