@Ollandeza here are some tips, i've found help me make reasonably realistic ponds
1.) slow down the terrain manipulation speed to the slowest speed and soften the brush the whole way. (the top slider should be all the way to the right, the bottom one should be all the way to the left) it takes a lot longer but you dont get what i call the "cliff texture" and it looks more natural
2.) try using the flatten terrain to height tool rather than the raise/lower terrain ones. those two will keep lowering and lowering the terrain infinitely (well finitely considering theres a minimum height on lots, but it will keep going and going until it reaches that point).
start slow with a height only two or three increments lower that the height of the surrounding area. once you get that into a reasonable shape, try going another one or two lower than that in *a section* of the middle. unless youre planning on building a huge lake, its probably not going to end up nearly as deep as youd think a pond should be. (anytime i've tried building a deep pond, it very quickly starts looking like just a hole in the ground)
3.) design the shape and paint the ground to mud/sand/whatever-non-grass-texture-the-pond-will-have before you put water in (then make touch ups after you add it)