PBY diorama

I'm not sure what you mean by using "realistic water". If you mean what I think you want to build a display that wont damage your finished model. Perhaps you want to take it out of the display? For this the best method I've read about is using a product like Sculpey or Celluclay. You would mold this into the general shape you're looking for. Then I would suggest protecting the bottom of your finished model with some sort of masking material and using the model to make an impression into the base before it dries. If you're concerned about damage to the model, you may want to consider buying a second kit and building a master for the purpose of making impressions. There have been some very good articles in FSM over the years.

In the February FSM Rusty White describes the technique he used for making waves--starting with Sculpey putty--on his model of the USS Barney. The result is pretty darned convincing (take a look at the article's lead photo on pp. 48-49).

Have a look at WWW.woodlands.com it is a company which makes scenery for trains. They have a liquid which dries up like realy water.

Buy Verlinden Publications "Modeling, Detailing, Painting, Weathering WWII Aircraft" No. 1351. They did a beautiful diorama of a PBY in the water. The sharks are a little sick, but those are easily removed. You can buy this at Amazon.com.
Good luck
Dave