Purchase or build the model ship that will be used in the diorama. The ocean cannot be constructed until the ship is chosen -- a tiny ocean would look silly with a large ship taking up most of it. Decide on the model ship and construct the ocean around it.
Sketch the diorama in detail on a sheet of paper -- this will be the reference for constructing the diorama. The larger the diorama will be, the more details can be included. For example, a small shoebox diorama may just have a blue ocean and the model ship, but a large diorama can include a port, animals and plant life.
Paint the base of the diorama in one shade of blue or several shades of blue for extra dimension. Heavy strokes will create waves. Highlight waves with white paint to depict stormy waters.
Place small models or paint images of the ocean’s ecosystems on the diorama, including coral, lagoons, plants, fish and mammals. The specific location where the diorama is set may need to be researched to ensure that these details are accurate. If the diorama is reflecting a scene from a book, carefully read the book for a description of the setting.
Add a port to the diorama if the ocean’s shoreline is being depicted. Construct a wooden dock from toothpicks and a “cement” dock from a piece of painted foam. Include buildings and docked ships, and paint the port an appropriate color, like green for grass or tan for a sandy shoreline.