You might be able to do what you want to do with the fish locations provided you only post the locations associated with a user's Tag . If you cannot do it with run time markers you probably could code as many design time markers (use as many markers as any fisher might be allowed to post). Hide those markers any specific user will not use and display the needed Markers.
What have you tried Tolis? Have you created an app that would allow you to do this for just a single user yet?
Here are a few similar Projects being discussed (they do not use CloudDB) :