If it were me I would start with styrofoam. I would shape large chunks into the different parts that I wanted ( stump body and roots). Then I would assemble them together. After that I I would carve in the detail. Finally, I would use colored concrete to solidify the form and give it weight. I would go about it in pretty much the same way you would make a DIY background.
I have thought about this myself but I would think for someone to do this they would need to have some type of artistic ability which is why I have never attempted this myself lol
No, I don't think a stump would be easier. The good thing is that it is summer time and the water is warm. I think if you searched the waters a it you could find something good. You just need to look i the right places. Best of luck to you.