If you can't or don't want to buy premade items, you make it with whatever you have laying around and your creativity.
You can make dirt by taking real dirt and gluing it to a board with elmer's glue.
You can use small pebbles as realistic boulders.
You can make shrubs by taking moss and parts of real plants and gluing them down.
You can make a dock out of popsicle sticks and small twigs.
You can make a fishing pole out of a toothpick with a little bit of paint and drops of glue painted over.
Creating realistic water might be trickier without buying the kit (available at Michael's Crafts)
To make a boat, find something that can be carved and get to work. Even a stick of soap can work if that is all you have, but I would use a twig instead. When you get the shape you want, paint over it.
Sorry, I don't know anyone that can teach creativity
That is correct. Creativity can't be taught. You have to look at things differently and figure how they can fit into what you want to make. A piece of old door screen becomes a chain link fence, the ends of tan paint brushes become overgrown grass, etc. Go to the library and see if they have any books on military dioramas or model railroading. All those techniques can be used in little diecast worlds. Search on line. It's hard for someone to advise how to make a complete fishing scene. There would me many, many small details and techniques that could be utilized. Just remember. Have fun. It's a hobby.