Unreal comes with ocean and lake water blueprints. You can get them from the learn tab.
Water is tricky. Math and physics knowledge is advised. Anyway, you could take a look at Gameworks as well.
For your lake or ocean just create a mesh with a big hole in the middle where your lower than sea level will be.
You can apply water to any shape mesh it doesn’t have to be a box.
Thank you all. I appreciate the help.
There’s already a large amount of examples pertaining to water, oceans, lakes and so forth. At the most basic level, it is a mesh (plane, typically) with a shader applied. As others have noted, Epic’s already released sample content focusing on this. I believe it can be found under the “Learn” tab in the Epic Games Launcher.
As a note, those who are looking to learn how to create materials and shaders in Unreal Engine 4 might want to take a look at 3DMotive’s recent tutorial series on this. It’s located here. There’s probably free alternative tutorials on YouTube if you dig for them a bit.