Under what circumstances is an operator involved in a boating accident NOT required to stop his or her vessel immediately at the scene of an accident and render assistance?

46 Code of Federal Regulations-section 2304
"A master or individual in charge of a vessel shall render assistance to any individual found at sea in danger of being lost, so far as the master or individual in charge can do so without serious danger to the master's or individual's vessel or individuals on board." Failure to comply can result in criminal charges.

Unless the scene is immediately hazardous to yourself, your passengers or your vessel, you should, as prudent mariner, render assistance whenever possible.