After conducting research, I could not find any information regarding a boat ramp named 6th Street Northwest Mercer County in North Dakota. It is possible that the information provided in the question is incorrect or outdated. Therefore, I cannot provide any accurate details regarding the kind of boat ramp it is, its width, the body of water it services, or the permitted craft on the water.

However, I suggest reaching out to local authorities or organizations, such as the North Dakota Game and Fish Department or the Mercer County government, for more information on the specific boat ramp in question. They may have more accurate and up-to-date information on the facilities, services, and regulations surrounding the boat ramp.