This is part of a new project that I have started, I thought it could be useful for you.
1- They open a ditch and flatten the ground in a staggered way starting from the top down. Leave the sea closed so that it does not fill with water while they work.
2- Color to your liking and place some stones.
To place objects on large surfaces I use two forms.
-Quickly place at random while f repeatedly. This option is the most realistic.
- Make a section and copy the elements when you paste in another place, use the option to adjust the terrain. This can be done with the whole set of objects but you will have to retouch if you do not want the repetitions to be noticed
3-Place items such as oyster shells or corals and paint some stones.
4-Implement the vegetation that you like the most and some decorations like drift boats or withered trees.
5- add water and enjoy its beautiful river
PS: Some sharks ate beans...