Whirlpool!!! Yuck KUDS21MSO
Well, it is not a built by Hobart Monterrey..It is one that was built after the buy by Whirlpool. The KUD gives it away. Hobart never used "U" designation and WP did. But be that as it may,it wasn't until the 23 series machines that WP changed the pumps to their Power Module pump system. Your unit has the metal disposer/drain impeller and like Gansky said, none of the pump workings can be seen from opening the door or removing the washarm. It has to be disassembled to see what is the problem.
But first, make absolutely sure that the drain the hose drains into is not clogged or sluggish. There is only so much resistance any dishwasher can pump against and if it is too much, the unit(no matter what brand) will not be able to pump all the water out.
I suggest you get a 5 gallon pail and disconnect the drain from the sink and let it drain into the pail. If all the water leaves the machine, you know your problem is in the plumbing.
Since you said the lines are clear, then this would seem to be the most likely scenario. These units had very well protected pumps and drains and it is really unlikely that a coin would work its way into the pump without some really horrendous noise coming from the drain system.
If and when you disassemble the pump, you will see that the metal drain impeller was actually part of a miniature garbage disposer which really ground up just about anything sent its way. They don't usually fail to drain out. The typical failure is the seal just fails and it leaks water.
Now there could be some kind of paper or plastic wrap all inside preventing the water from draining, so a pump breakdown is probably a good idea too. Just remember where everything comes out of and return it back exactly as you found it including the rubber o-ring on the shaft as well as the shims you will find there. They must be put back in their correct locations as some shim the botton drain impeller and some shim the wash impeller and they are not the same outside diameter so you can mess them up and have worse problems.
Just take your time and you will succeed. If you want a breakdown of this, let me know and I will scan and email it to you.