It seems obvious that you would not want anything permanent. Anything sturdy enough to support a person would distract from the aesthetics of your pond if left there. You don't have the space to create a fancy arch bridge over this pond. It would hide the whole pond. I think the ladder idea is probably about the best. Use a short extension ladder (just long enough to reach the distance), place a few decorative flat rocks on each side to rest it on so that it will be level, then lay a 1" x 12" plank over the ladder steps to walk on. Remove it when you are done with your maintenance and store it behind the shed or in the rafters of your garage. The ladder and plank should be sturdy enough for almost any person and light enough to contend with.
I wouldn't use a soft pine for the plank, but maybe a Birch or Cedar.
Catfishnut