What condition would require that a vessel carry at least one Type B fire extinguisher on board?

By themselves, none of these conditions require a fire extinguisher on a boat:

• outboard motor
• passengers
• fixed, unenclosed seats
• over 16 feet
• operating at night

Also, a vessel less than 26 feet would not require a fire extinguisher unless other conditions are present (inboard motor, fuel tanks, etc).