Patsy, your fish cake sounds like fun. I have done some cakes with lakes or streams. Here is how I made the water.

Put a somewhat thick layer of white (or very pale blue) frosting over the area you want to have as lake. Maybe 1/4 or 1/2 inch thick. This does not have to be perfectly flat. Cover the white frosting with a layer of blue piping gel. The piping gel also does not have to be flat. Then, take a narrow knife or spoon handle and dip it into the white frosting, and pull it up and swirl it through the blue piping gel. This give the illusion of waves. Do this as much as you wish depending on the roughness you want on the water.

I usually make the side of the cake with chocolate frosting and chocolate-covered candies, raisins, nuts, etc. .... as if it is Earth. Of course, a touch of green grass is a nice touch here and there by using a very thin lettering tip with green frosting. Gummy worms can even be crawing from under the cake or candy.