So how could I get over that jagged edge? Covering it with rocks would be unrealistic since it is a waterfall. Would plants help?

Hold Shift and put the Water Tile again and cover that, pressing Shift will let you put any Doodads/Destructibles without Pathing Size but it has on-game, it's just a good way to put doodads anywhere you like

Yeah I know that, sadly that's not the problem since the custom water doesn't have pathing, and if I put more to cover that, the water would be like sticking out on the edge of the cliff.

Is that on the Top Island by the way? If yes, don't mind it because I do know this is for a showcase or no? If it's not noticeable in showcases, better leave it as it is.

It's not at the island, it's at the waterfall leading down the hill..
However, it's not only for showcase, I plan to use it as playable terrain(with camera rotates possible).

Make the water doodad that you will put there smaller so that it fits... and those water can use better sizing and placing, right now I think ur using the default size and I can see the edges between each water tile...

Oh.. thanks, I never thought of putting multiple waters in a row.

Rocks could be nice.

Isn't that... The teeth of DOOM! It was once said to singly bite the titan PROmeatyuz in half! It's return has been foretold, they never listened... [music="dovahkiin"]Teeth of doom, teeth of doom, will bite your ass off! Want to fight, you won't fight...[/music]

O.O What?

lower a bit under the jagged edge and place the waterfalls closer to the water so they bind together. Also you can cover things with rocks and plants or even with raising the terrain if you want to.