Do you have any woodworking tools and/or skills?
Arrange your display on a sheet of paper, then transfer it to the wood before cutting it. It is easier to cut it smaller - more difficult to cut it bigger.
I think I'd use some plywood 3/8" or 1/2". Perhaps some MDF. You may want to band it with some veneer or solid wood to cover the raw edge. Rout a decorative edge on the banding, or at least chamfer it to ease the edge & minimize breakout.
Sand, stain or paint, and finish the wood before building the water. When dry mask the edges to protect them from the application of the diorama material
Build up any land details and rough paint them
Lay in the first base-coat of water. Imbed the ships and other water-based details. If your are doing something like Celluclay for the water, paint it.
Lay on washes of colored gloss to make the water up to the desired level. Go around the ships. This will tie them into the diorama. They are now permenantly affixed. To late to move them. Lay the water up to the lands edge.
Finish detailing the land and model items