Actually you do lose water during the cooking, unless the design of pressure cookers has changed fundamentally from that of my 30-year-old Prestige, that is. It escapes as steam when you're up to pressure, hence the hissing noise from the spring- or weight-controlled valve.

Mind you, the only things I pressure cook are stock and pot roasts, where the liquid is an essential part of the recipe. For other stuff I would probably use the trivet as Doug suggests.