The standard vegetarian way to feel full is with fibre which also sort of holds water without it swooshing around in your stomach. All plant foods contain fibre. If you don't want to consume excess calories, the way to do it is with the lowest calorie plants, the non-starchy vegetables. Really what you want, for a host of reasons, is to eat about a pound of leafy greens a day. They are low carb, even as a percentage of their low calories, high protein, contain some omega fats too, and are rich in many nutrients.