Actually, the units used depends on where you are sailing. In the Great Lakes system and (I think) the Mississippi as well, miles and mph are the correct units as the charts are laid out in miles. Coastal and off shore is nautical and knots.

 

Never thought of it until a court case where the lawyer was questioning a vessel captain in a Great Lakes wreck case and repeatedly asked him how many knots he was traveling. He repeatedly said he didn't know. Finally the lawyer asked him how he could not know how fast he was going. He answered that he did know and stated the mph. He also said he had no idea how to convert to knots.

 

True story

 

Rob