We have about a 15x25 or less rectangular pond (built to be a pool), and our pump and filter (Savio Bio, very skippy-like) seemed to cause no dead spots in the pond.
However, now I have added a TON of very LARGE water hyacinths. I let them float to where they wanted, and they seem to congregate in certain little areas. It seems the surface is very still behind them. Did they create a dead spot? (The pump is on the bottom in the middle.)