2002 Merc 125hp Serial #0T538993

I had some issues starting my motor while fishing this weekend. When I turned the key it sounded like the starter was spinning but not engaging the flywheel. I pulled the cowl and confirmed this. Then I spun the starter with my fingers and when I turned the key again it fired right up. I had to do this a few more times throughout the day but I was able to get it going each time. My starting battery was fully charged and working perfectly, no problems there. Do I need a new starter or is there something I should check first?

If I do need a starter, is it as easy to replace as it looks? I'm not a mechanic but consider myself handy with tools and not afraid to pull things apart.