Yes, you can do it. I'm not sure plaster would be the best choice though. It may dissolve easily. Concrete would be a better choice, but its very heavy. A glass divider is the most common way that this is done. One side is a pool and the other is filled with sand or a substrate of your choice.
What type of fish do you have in mind? One fish that might be good for your setup is an archer fish if you have an air tight canopy. Archer fish shoot down insects, such as flies by spitting water at them and eating them. They really need a tank with open air so that they can shoot down the insects.
Some other good things to try are freshwater crabs and mudskippers. They both need land and water.
Some people have used silicon glue to hold a peice of glass half the height of the tank as a tank divider to hold substrate on one side and water on the other. It is a good place to start, but doesn't look very natural. It appears as a steep cliff into the water. So you might add some rocks or other items to give the cliff a slope.
One last thing to consider is that you will need a large tank to do this. At most, probably 30-40% of the tank will be filled with water. A 10 gallon wouldn't be a good choice. I would stick to a 55 gallon or larger tank.
You can also look for a commercial kit to do this. I have seen one adverstised at "that fish place" several years ago. It comes with a plastic terrain outline and a water pump. Just a warning though, be prepared to pay for it. I think it was over $60.