What rating can I add to a commercial certificate with the least time/effort?
Jack Brown's Seaplane Base offers single engine seaplane ratings at private and commercial level.  It takes 2 full days and costs between 1400 USD and 2300 USD depending on your choice of training aircraft.  

From their website:  


  SES Rating
  
  We offer three options of planes for your SES training at the private
  or commercial level, both of which include: 1.5 hours of ground
  instruction, 5 hours of dual instruction, the check ride and the
  examiner’s fee.
  
  Training in the Piper J3-Cub on floats,  1,400.00.
  
  Training in the Maule M-7 on floats,     2,300.00.
  
  Training in the Super Cub on floats,     1,850.00.
  
  If additional flight instruction hours are required they are billable
  at the hourly rate of the aircraft (listed below).


They are located in Winter Haven, FL.