Water is meant to drain almost completely from a dishwasher tub. A bit of water may remain in the bottom after a cycle, but any more than a cup or two means there is a problem with the drainage system. Because dishwashers are hooked into the rest of the kitchen plumbing, the problem may reside there rather than within the unit itself. Your best solution is to check each area for clogs and perform regular maintenance to avoid the problem in the future.
Check for clogs in the dishwasher drain opening, kitchen sink drain and air gap. Put on rubber gloves and reach into the back of the dishwasher. Remove any debris in the drain opening. Remove the U-bend from beneath the sink with a pipe wrench and remove any clogs. Pull off the metal cover next to the sink faucet and remove the plastic cover underneath. Clear clogs and replace all parts. Repeat monthly.
Check the garbage disposal drain plug beneath the sink if you recently had a garbage disposal installed. Remove the plug if it is still intact.
Pour 1 to 2 cups white vinegar into the bottom of the dishwasher tub. Run the hottest, longest cycle to descale the water lines. Repeat monthly.
Reduce the amount of dish soap used for each load and make sure you are using a type of soap approved by the manufacturer. Load the unit according to manufacturer instructions. Be sure not to block the drain area with pots and pans or other large dishes.