I've got some questions about "waterline".

It appears that the bigger boats need a lot of ballast in order to sit down in the water properly, regardless of how heavy they are. Does this make them more stable? If I run a flat-bottomed hull (instead of full depth) and run only a few inches down, will my boat be unstable? (Tip over?). I want to go the easiest route but I want it to work. I don't care what it looks like out of the water; I only want it to look real from the water up. Is this flat hull a bad idea?