To make the pond shape, start by subdividing a plane (the less the better - you will see why later). Make your rough "splat" shape by selecting the faces that won't be part of it and deleting them.
Because the plane is subdivided, it will become "blocky looking" if and when we add a Subdivision Surface Modifier, so the first step is to reduce the subdivisions with a Decimate Modifier set to Planar. (This is also why it is better to have less subdivisions, because the points around the edge contribute to the interpolation of the edge (or lack thereof) and will also cause unwanted blocky-ness - fewer points means smoother curvature )
Next add a Subdivision Surface modifier to smooth out the edges as just discussed:
For the material, I suggest using this setup for your water surface. The "depth" of the water can largely be altered by changing the IOR of the Refraction Shader. Don't forget to change your Blend Mode if you're using EEVEE. I also notice after I uploaded the image, the top Node that serves as the Mix Factor between the Diffuse and Glossy shader is cut off - it is a Fresnel Node.
You can see as well that I made the surface by using a Noise Texture, with it's effects softened by being mixed with white using a MixRGB Node_ before connected to a Bump Node and then a Displacement Node. The settings for the Noise Texture I left largely at the default, but play around and see what kind of surface looks best.
If you want a more "real " extruded look to the surface, you can actually select the whole plane and extrude it downwards slightly if you don't mind the extra geometry.
Here is a final result (lit with an HDRI). In this example, I increased the mix factor with white on the displacement to further soften the surface to create a more realistic look. (This can also be achieved by changing the Detail and Roughness on the Noise Texture, as well as controlled somewhat by adjusting the strength or the distance of the Bump Node - play around and see what looks best for you).