You have to create a item or block with the and a material at minimum from what I had to do.
You will also have to edit the Biomes.xml and edit that with the new block as a resource.
Now if you want to just have the resource as a chance when harvesting or mining its easier to add it as a item and make it harvest in and add it to a block for example like this
Just really depends on how you want it in the game basically.
I added swimming fish to the water along with underwater plants and objects that are generated on the map to make the water more realistic feeling especially with fishing.