It may be a good idea to have the starter looked at by a reputable starter-alternator shop as many can refurbish small engine starters. Some dealers may also service starter motors, though some just want to sell you a new one. Rebuilt starters are available also.
With yours being 15 years old, it could be needing new brushes, the commutator machined, and the bushings replaced or lubed as needed.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.