PS - As an alternative if you do not want to use nurbs curves and lofts which tend to scare people. Start with a 1x1 poly plane and extrude edges from the top view to form the upper rim / lip. then just extrude the inner edges down and adjust them from the top view to form the tub, add support edge to get the desired corner tension, and finally just fill the hole at the bottom and use split polygon to quad up the ngon if you feel you must bow to the all quad god.