I made my fiancee, Tom, a fishing themed cake for his birthday in 2011. I made the components for the cake using gumpaste and molds. This was my first time using molds and I have gotten much better at it since. I was really happy with how it turned out. I've learned that when you are trying to judge the size of the pieces that are going on the cake before you've baked the cake, there is a lot of room for error. It is a lot harder than you would think to get everything the right size. I do have a point here...my fish was way too big. The only place to buy molds where I live is the internet and sizes aren't always clear, unfortunately. Here are the components that I made to go on the cake.
I used pastel green and white marbled fondant on the bottom tier and pastel blue and white marbled on the top tier, then blended them together using frosting in light blue and white to look like water. Around the bottom I added grass and things that you would see growing out of the banks of a lake.
He loved it and I was pretty happy with it too!